SEOUL, South Korea — South Korean troops pushed ahead with naval firing drills Monday, a day after North Korea warned the exercises would aggravate tensions between the rivals following the North’s deadly shelling of a front-line South Korean island.

Regional powers stepped up diplomatic efforts to head off further conflict, with President Barack Obama speaking to China’s Hu Jintao by telephone Monday and top diplomats from the U.S., South Korea and Japan scheduled to hold talks later in Washington.

Obama condemned North Korea’s Nov. 23 artillery shelling of Yeonpyeong Island and its pursuit of a uranium enrichment program, and urged Hu to send Pyongyang a message that its "provocations are unacceptable," the White House said.

The attack killed two marines and two civilian construction workers — the first attack on a civilian area since the 1950-53 Korean War.

North Korea said it unleashed the artillery fire after Seoul went ahead with routine live-fire drills despite Pyongyang’s warnings to call off the military exercises. The North disputes the maritime border drawn in 1953 by U.N. forces, and considers the waters around Yeonpyeong, which lies just seven miles (11 kilometers) from its shores, its territory.

The shelling occurred just days after reports that North Korea had revealed a large uranium-enrichment facility that would give it a new method of making material for atomic bombs in addition to its known plutonium-based program.

On Monday, the South Korean army launched a new round of artillery exercises set to continue through Sunday, army and Joint Chiefs of Staff officials said.

The previously scheduled drills were to take place at some 30 sites off the coast throughout the week, but there were none Monday along the disputed western sea border, the officials said, speaking on condition of anonymity according to department policy. Warships will join the drills later in the week, the navy said.

Similar live-fire drills will take place next week as well, officials said.

North Korea warned Sunday that the drills were causing "uncontrollable, extreme" tension on the peninsula.

"The South Korean puppet group, far from drawing a lesson from the deserved punishment it faced for its reckless firing of shells into the territorial waters of the (North Korean) side around Yeonpyeong Island, is getting more frantic in military provocations and war moves," the North’s official Korean Central News Agency said. Seoul said its forces fired away from North Korea in last month’s drills.

On Monday, President Lee Myung-bak, acting on criticism that troops acted too slowly and too timidly to the North Korean attack, pledged to reform the military.

"The South Korean people believe our military has to change," Lee said at a Monday meeting on military reform, his office said. "What the military needs now is (increased) mental strength."

National security is at a "critical juncture," Kim Kwan-jin, South Korea’s new defense minister, told reporters. He said he would focus on boosting the military’s morale and discipline. Kim was appointed after the previous defense chief resigned in the wake of the shelling.

South Korean Prime Minister Kim Hwang-sik also announced 30 billion won ($27 million) to help rebuild shattered Yeonpyeong Island. Most of the 1,300 civilians who lived on the island have fled and many are living in a public bathhouse in the port city of Incheon that has been converted into a refugee center.

Elsewhere in Incheon, the bodies of two construction workers killed in the attack were cremated as their relatives screamed and cried.

"I sincerely hope that officials take care of us so that this kind of thing will never happen again to innocent, weak and poor people," said Kang Sung-ae, widow of dead construction worker Kim Chi-baek.

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Associated Press Television News cameraman Yong-ho Kim in Incheon contributed to this report.