TOKYO, May 30 -- North Korea rattled its neighbors again Friday when it fired a short-range missile off its eastern coast, the sixth such launch this week, and there were signs that the secretive communist country might be preparing more shows of force.

The latest launch appeared to be of a modified version of a Russian SA-5 missile designed to shoot down aircraft, according to South Korea's Yonhap news agency. Yonhap also said that the usual fleet of nearly 300 Chinese fishing boats in the disputed waters west of the Korean Peninsula had dwindled by about half in recent days. It was not known whether North Korean officials warned the Chinese boats to move or they left on their own, hoping to avoid clashes in the area.

A South Korean newspaper reported Saturday indications that the North is preparing to transport a long-range missile to a launch pad in the country's northeast. The mass-circulation Dong-a Ilbo paper said U.S. spy satellites had detected the preparations, adding that it would take about 15 days for North Korea to move the missile to the pad by train.

At an Asian security meeting in Singapore, Defense Secretary Robert M. Gates told regional defense ministers, "We will not stand idly by as North Korea builds the capability to wreak destruction on any target in the region or on us," the Reuters news agency reported.

Gates also delivered a stern warning to North Korea against selling or transferring nuclear weapons or material outside its borders.

Such actions "would be considered a grave threat to the United States and our allies, and we would hold North Korea fully accountable for the consequences of such action," he said.

The latest missile firing came after the joint command for South Korean and U.S. forces on the Korean Peninsula raised its alert to the second-highest level, Watch Condition II, in response to an extraordinary week of truculence by North Korea, beginning Monday with an apparent underground nuclear test.

The joint command, which has 655,000 troops from South Korea and 28,500 from the United States, last raised its alert to this level in 2006, after North Korea first tested a nuclear device.

Gates said Friday in Guam that he saw no immediate need for more U.S. troops in South Korea.

"I am not aware of any military moves in the North that are out of the ordinary, at least," Gates told reporters. "I don't think there is a need for us to reinforce our military presence in the South."

However, South Korean Defense Ministry spokesman Won Tae-jae said that "surveillance over the North will be stepped up, with more aircraft and personnel mobilized."