President Trump canceled plans Tuesday for his first trip to South America, in another sign that he is preparing to order an imminent military strike against Syria over its deadly chemical weapons attack against civilians in a rebel-held suburb of Damascus.
Syrian government forces were on high alert and taking precautionary measures at military positions across the country. The Britain-based Syrian Observatory for Human Rights said that Syrian government troops were on a 72-hour alert and were fortifying their positions.
Adding to the tensions, Iran, a strong ally of Syrian President Bashar Assad, threatened to respond to an airstrike on a Syrian military base on Monday that the Syrian government, Russia and Iran blamed on Israel.
The president consulted other world leaders about holding Mr. Assad accountable for the atrocity Sunday that killed dozens in Douma, a rebel-held suburb of Damascus. The emir of Qatar, meeting with Mr. Trump in the Oval Office, said he and the president agree that Mr. Assad is a war criminal whose continued rule cannot be tolerated.
The White House said Mr. Trump decided to remain in the U.S. this week rather than travel to the Summit of the Americas in Peru on Friday so he can “oversee the American response to Syria” and monitor other developments around the world. Mr. Trump has promised a “forceful” response in Syria.
Syria and its military ally, Russia, have denied any chemical attack, but White House press secretary Sarah Huckabee Sanders said the U.S. is confident with its judgment that Syrian forces used a banned chemical weapon in the attack.
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“We’re working with our partners and allies and our national security team to look at all options,” she said. “I’m not going to get into specifics on intel matters and things that we may or may not do.”
As the U.S. prepared its response in Syria, Russia reportedly was jamming some U.S. drones over Syrian airspace, seriously impacting military operations, four U.S. officials told NBC News. Officials said the jamming operation began several weeks ago.
In response, Sen. Ben Sasse, Nebraska Republican and a member of the Senate Armed Services Committee, accused Russian President Vladimir Putin of waging a “shadow war” against U.S. forces in Syria.
“It is insane to think that Russia is anything but an adversary,” Mr. Sasse said. “Americans want to kill ISIS and prevent the normalization of chemical weapons attacks against innocent families. Russia wants to undermine our interests at every turn.”
Russia on Tuesday vetoed a U.S.-drafted United Nations Security Council resolution to set up an expert body that would assign blame for chemical attacks in Syria. Twelve council members voted in favor, and Bolivia joined Russia in opposition. China abstained.
“This resolution is the bare minimum that the council can do to respond to the attack,” U.S. Ambassador Nikki Haley told the council before the vote.
Russian Ambassador Vassily Nebenzia asked the Security Council, “Why do you need this mechanism when you have already appointed the guilty party before the investigation?”
U.N. Secretary-General Antonio Guterres said he was outraged by the attack and that the use of chemical weapons would violate international law.
The international chemical weapons watchdog said it would send a fact-finding mission to Douma after receiving a request from the Syrian government and its Russian supporters to investigate. It was not immediately clear whether the announcement by the Organization for the Prohibition of Chemical Weapons would delay or avert U.S. military action against Syria.
Vladimir Shamanov, a retired Russian general who heads the defense affairs committee in the lower house of parliament, said in televised remarks that a U.S. strike in Syria could hurt Russian service members and trigger retaliation.
He said Russia has “the necessary means for that, and the Americans and their allies know that quite well.”
Mr. Trump spoke Tuesday with British Prime Minister Theresa May and French President Emmanuel Macron, who said France would decide in the coming days on “a strong and joint response” with the U.S. and Britain to the suspected attack. A British government spokesman said that although Britain was still awaiting confirmation of a chemical attack, Ms. May and Mr. Trump “agreed that the international community needed to respond to uphold the worldwide prohibition on the use of chemical weapons.”
With those high-level talks proceeding, Mrs. Sanders said, the president will not attend the eighth Summit of the Americas in Lima, Peru, or travel to Bogota, Colombia, as planned. It is the first time a U.S. president has skipped the summit. Vice President Mike Pence will attend in his stead.
“The president would like to stay in the country while there are a lot of developments going on around the world,” Mrs. Sanders said.
The president’s new national security adviser, John R. Bolton, urged Mr. Trump to skip the trip, an official said. This reflects a view in the White House that deeper Russian and Iranian involvement in Syria have complicated calculations about a response to any U.S. military attack, according to the official, who spoke on the condition of anonymity to discuss internal deliberations.
In his Oval Office appearance with the emir of Qatar, Mr. Trump made no mention of his intentions in Syria. Sheikh Tamim bin Hamad Al Thani said he and Mr. Trump “see eye to eye” on Syria.
“We cannot tolerate with a war criminal,” he said. “This matter should end immediately.”
The U.S. military appeared to be in position if Mr. Trump orders an attack. A Navy destroyer, the USS Donald Cook, got underway in the eastern Mediterranean on Monday after completing a port call in Cyprus. The guided missile destroyer is armed with Tomahawk cruise missiles, the weapon of choice in a U.S. attack one year ago on an airfield in Syria after a sarin gas attack on civilians.
The Navy also said that the USS Harry S. Truman aircraft carrier and its strike group will depart Norfolk, Virginia, on Wednesday for a regularly scheduled deployment to Europe. The Navy does not currently have a carrier in the Persian Gulf.
Seven Iranians were among the estimated 14 people killed in the Monday missile strike on a Syrian base, and a senior Iranian official visiting Damascus said the attack “will not remain unanswered.” Ali Akbar Velayati, an aide to Iran’s supreme leader, spoke upon arrival in the Syrian capital on Tuesday.
The Syrian air base was struck by missiles a little more than 24 hours after the chemical attack. Israel does not typically comment on its operations in Syria, and it is unclear whether the missile attack was linked to the suspected use of chemical weapons.
Iran is one of Mr. Assad’s strongest backers and has sent thousands of troops and allied militia members to support his forces.
• Sally Persons contributed to this article, which is based in part on wire service reports.