SEOUL — South Korean activists floated balloons carrying tens of thousands of anti-Pyongyang leaflets into North Korea on Monday, eluding police who had disrupted an earlier launch attempt.
North Korea's military warned last week that it would strike if the South Korean activists carried through with their plan to fly balloons carrying the propaganda leaflets across the border.
South Korea pledged to retaliate if it is attacked.
South Korean police, citing security concerns, had sent hundreds of officers Monday to seal off roads and prevent the activists and other people from gathering at an announced balloon launch site near the border.
Residents in the area also were asked to evacuate to underground facilities, according to local official Kim Jin-a.
Later in the day, some of the activists, mostly North Korean defectors, moved to another site near the border that was not guarded by police and carried out the launch of the balloons.
South Korea's Defense Ministry said it was closely monitoring North Korea's military movements, but there were no suspicious activities.
Before taking action Monday, the South Korean government had implored activists to stop their campaign, but had cited freedom of speech in not making further attempts to intervene.
South Korean activists in the past have sent leaflets across the border, and North Korea has issued similar threats to attack without following through.
But Seoul's Yonhap news agency reported Monday that the ban on entering the border area was imposed as South Korea detected that North Korea had removed artillery muzzle covers and deployed troops to artillery positions in possible preparation for an attack. Yonhap cited no source for the information.
Defense Ministry spokesman Kim Min-seok told reporters Monday that North Korea was believed to have acted in line with carrying out its threat. He declined to elaborate on the North's army movement as that was confidential military information.
He said South Korea had bolstered its military readiness following the North's threat and would "strongly" retaliate if attacked.
The activists said they floated balloons carrying about 120,000 leaflets critical of North Korea's young leader Kim Jong-un and his country's alleged human rights abuses. They said they wanted to let North Korean people know the true nature of their country.
"We could not delay our plans to send anti-North Korea leaflets because it is our love toward our northern brothers," the activists wrote in a statement posted on the website of Seoul-based Free North Korea Radio, one of civic organizations involved in the leafleting.
Lead activist Park Sang-hak had said the ban on entering the border area was tantamount to yielding to Pyongyang's threat. "It's surrender. It's clearly surrender," he said.
On Monday, the top U.S. envoy on North Korea urged Pyongyang to stop issuing destabilizing threats.
"It is grossly disproportionate to have threatened to respond to balloons with bombs," Glyn Davies told reporters in Beijing after meeting with Chinese officials.
China, the North's main ally and biggest aid source, welcomed South Korean efforts to quash the balloon-flying and urged all parties to exercise restraint.