Secretary of State John F. Kerry on Sunday urged Iraq not to let Iran use its airspace to supply weapons and fighters to Syrian President Bashar Assad's regime.
The Syrian opposition, meanwhile, was plunged into crisis as its leader quit after expressing frustration with a lack of support for the rebels from the international community.
The 2-year-old war in Syria spilled over the border Sunday as Israeli troops in the Golan Heights destroyed a Syrian military machine gun position after coming under fire from that area. Israeli Defense Minister Moshe Yaalon blamed the Syrian military for the incident.
Iran's Shiite regime has provided a steady stream of military support to Mr. Assad even as the international community has debated the wisdom of arming the Syrian opposition.
More than 70,000 people have been killed in the uprising against the Assad regime and more than 1.1 million refugees have fled to Syria's neighbors, the U.N. says.
The Obama administration, alarmed by the support Mr. Assad receives from Tehran, wants the Iranian flights through Iraqi airspace to stop or, at the very least, be inspected in Iraq to ensure that they are carrying humanitarian supplies.
Iraqi Prime Minister Nouri al-Maliki's government last year promised then-Secretary of State Hillary Rodham Clinton that it would inspect the Iranian planes.
Only two planes have been inspected since July, said a senior U.S. official who spoke on background. In both cases, Iraqi inspectors found humanitarian supplies.
Mr. Kerry said Iranian support has sustained the Assad regime and that he had a "very spirited discussion" with Mr. al-Maliki on this subject when they met in Baghdad.
Urging Mr. al-Maliki to put an end to such support, Mr. Kerry said some members of Congress and the American people are "wondering how it is that a partner for whom Americans feel they have tried so hard to be helpful ... can be, in fact, doing something that makes it more difficult to achieve our common goals."
The Iranian flights to Syria occur "close to daily," said the senior U.S. official who spoke on background. "It goes up and down. But it's substantial."
The official, who spoke before Mr. Kerry's meeting with Mr. al-Maliki, said the secretary of state would "be very direct with Prime Minister Maliki about the importance of stopping the Iranian overflights and the transits across the territory, or at minimum inspecting each of the flights."
"And his goal is not to get into a tit-for-tat about how we know this or how we know that, but to explain that the number of the flights is, in itself, an indication that these can't possibly be only humanitarian flights and that he, himself, as secretary of state, is convinced that they include weapons and fighters and that this is absolutely contrary to the international goals with Syria and is dangerous for Iraq," the official added.
On Sunday, the Western-backed Syrian opposition coalition was dealt a blow when its leader, Moaz al-Khatib, resigned saying the international community has not done enough to support to the rebels.
"I promised the Syrian people and God to resign if matters reach some red lines," Mr. al-Khatib said in a statement announcing his resignation as head of the Syrian National Coalition. Mr. al-Khatib was chosen to serve as president of the coalition when it was formed in November.
He said he resigned so he could "work with more freedom unavailable to me within the official position of the organization."
Mr. al-Khatib's frustration with international community was evident in February when he expressed a reluctance to attend a Friends of Syria conference in Rome. He eventually did so after pleas from Western officials, including Mr. Kerry.
Mr. Kerry said on Sunday that he was sorry to see Mr. al-Khatib go, but U.S. support for the opposition would continue.
"We view this as a continuum," he said. "It's not about one person It's about an opposition that is bigger than one person."
Mr. al-Khatib had often expressed his wish to step down, but was urged by supporters within the opposition coalition to complete his term in office, which ends in May, or at least stay for the Arab League summit in the Qatari capital, Doha, this week.
Najib Ghadbian, who represents the Syrian opposition coalition in the U.S., said Mr. al-Khatib's resignation was not unexpected but, nevertheless, "not the best news."
Opposition leaders will ask Mr. al-Khatib to stay on, but in the event that he insists on his resignation, one of the coalition's three vice presidents will be named acting president until a fresh election is held, Mr. Ghadbian said in a phone interview.
The Syrian National Coalition rejected Mr. al-Khatib's resignation.
The opposition suffered a second setback when the head of its military branch, Gen. Salim Idris, refused to recognize the coalition's new prime minister. U.S.-educated Ghassan Hitto was elected last week to lead the opposition's interim government.
Gen. Idris said Mr. Hitto did not represent most anti-Assad groups. Other critics of Mr. Hitto's say he has been away from Syria for too long and out of touch with the country.
While the opposition has struggled to remain a cohesive force, the Assad regime has shown no signs of crumbling.
Last week, the regime and opposition accused each other of using chemical weapons.
Syrian opposition leaders said the regime had attacked Khan al-Assal, north of Aleppo; al-Otaiba, near Damascus; and the Baba Amr neighborhood of Homs city, with Scud missiles carrying the chemical weapons. They said 26 people had been killed and hundreds of others wounded in the attacks.
U.S. officials say they have no evidence that chemical weapons were in fact used. The United Nations in investigating the incident.
President Obama has said that the Assad regime will have crossed a "red line" if it uses chemical weapons. The administration has said this violation would result in serious consequences, but has not said what those might be.
On Sunday, Rep. Mike Rogers, Michigan Republican and chairman of the House Permanent Select Committee on Intelligence, said the U.S. must play a bigger role to prevent chemical weapons from falling into the wrong hands.
The U.S. should create a "safe zone" in northern Syria, Mr. Rogers told CBS' "Face the Nation."
"It means small groups with special capabilities re-engaging the opposition so we can vet them, train them, equip them so they can be an effective fighting force," he added.
The Obama administration is providing nonlethal aid to the opposition, including $60 million in aid to the Syrian National Council to help administer rebel-held territories and food and medical aid to rebel fighters. But it does not support a plan backed by Britain and France to arm the opposition.
The Syrian opposition wants the U.S. to do more.
"This is a very critical stage of the struggle, and we haven't seen much leadership on the part of the president," Mr. Ghadbian said. "Nobody is calling for military intervention, but in the area they call 'non-lethal' there is a lot of room for communication assistance and intelligence sharing."
"We believe that the best way to end this war is to speed up the process of overthrowing the regime," said Mr. Ghadbian. "We don't see that happening."
Sen. John McCain, Arizona Republican, said the international community, and the U.S. in particular, bears some responsibility for the setback to the opposition.
Mr. al-Khatib's resignation "represents a failure of American leadership, which is only further weakening what is left of Syria's responsible, democratic opposition," Mr. McCain said. "If the United States remains on the sidelines of this conflict, the Syrian revolution will be hijacked by al Qaeda, and anti-American radicals will inherit post-Assad Syria."
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