SEOUL, South Korea - South Korea said a North Korean patrol boat entered its waters around their disputed maritime border yesterday, but the boat backed off after nearly an hour following repeated warnings. A senior American diplomat, meanwhile, cautioned Pyongyang that its bad behavior would no longer be rewarded.

The naval standoff came amid concerns that the North might try to provoke an armed clash in the area - the scene of deadly naval skirmishes in 1999 and 2002 - to stoke tensions already running high after Pyongyang's nuclear test and a series of missile launches last week.

The regime has also conducted amphibious assault exercises near the sea boundary and appeared to be preparing for more missile tests.

The intrusion occurred as Deputy Secretary of State James Steinberg was in Seoul to coordinate a united response to Pyongyang's belligerence. South Korean news reports said a delegation of senior U.S. officials was working on financial sanctions against Pyongyang.

Steinberg told South Korean President Lee Myung Bak that "North Korea would be mistaken if it thinks it can make provocations and then get what it wants through negotiation as it did in the past. The U.S. won't repeat the same mistake again," Seoul's presidential office said in a statement.

Complicating the situation, two American journalists headed to trial yesterday in North Korea's top court, on allegations they entered the country illegally and engaged in "hostile acts."

Some experts believe that the North is using the trial and its nuclear and missile tests for leverage in possible talks with the United States, and that it hopes to win concessions or much-needed economic aid.

The North Korean patrol boat crossed about a mile into South Korean-claimed waters near their disputed western sea border and turned back about 50 minutes later without incident after a warning from the South, South Korea's Joint Chiefs of Staff said.

It was likely chasing away Chinese fishing boats illegally catching crabs in the area, it said.

Pyongyang did not comment on the maritime incident.

North and South Korea technically remain at war because they signed a truce, not a peace treaty, in 1953. North Korea disputes the U.N.-drawn maritime border off its west coast and has positioned artillery guns along the west coast on its side of the border, according to Seoul's Yonhap news agency.

Curtailing the North's financial dealings with the outside world is being considered as part of U.N. punishments, along with freezing company assets and enforcing an arms embargo, according to U.N. diplomats.