South Korea's president, Lee Myung-bak, has ordered the country's military to strengthen its troop presence on islands near its border with North Korea after this week's bombardment, as Pyongyang warned of retaliation for any "reckless military provocations".

The threat from North Korea came as a US aircraft carrier headed to the Yellow Sea for joint military manoeuvres with the South, due to begin on Sunday. A spokesman for China's foreign ministry said today that Beijing had "concern" about the drills.

The attack on Yeonpyeong island, close to the disputed maritime border, left two soldiers and two civilians dead and has heightened tensions on the region. It came days after reports that North Korea was operating a new uranium enrichment facility.

Lee pledged to reinforce troops in the area during an emergency meeting to assess the security and economic implications of Tuesday's clash. The government had been planning to scale down the military presence.

"We should not let our guard down in preparation for another possible North Korean provocation," Lee was quoted as saying by the Yonhap news agency.

North Korea did not mention the joint exercises in its statement, but said it would launch "strong physical retaliations without hesitation if South Korean warmongers carry out reckless military provocations".

The military statement, carried by state news agency KCNA, also blamed Washington for the live-fire exercises by South Korean troops that prompted the North to fire artillery at Yeonpyeong island, saying it should "thoroughly control" Seoul.

The US and South Korea are to press China to rein in the North, hoping that Pyongyang's closest ally will play a pivotal role in restoring calm.

But in Beijing's highest-level response so far, China's premier, Wen Jiabao, merely called today on all sides to exert "maximum restraint".

"China has all along devoted itself to maintaining the peninsula's peace and stability, and opposed military provocations in any form," Wen said, in remarks on the foreign ministry website.

The Chinese foreign minister, Yang Jiechi, has postponed a trip to Seoul this week, it emerged today. Seoul's foreign ministry blamed timetabling problems.

US state department spokesman PJ Crowley said last night: "China does have influence with North Korea and we would hope and expect China would use that influence."

Barack Obama earlier made it clear that he wanted Beijing to restrain North Korea, while Admiral Mike Mullen, chairman of the US joint chiefs of staff, said last night: "It's very important for China to lead."

Although the US is dispatching the carrier George Washington and other warships for a show of solidarity with South Korea this weekend, it has few options left to use as leverage on the North.

Washington had been due to take part in military training but had postponed involvement, citing scheduling conflicts.

Beijing had reacted angrily to plans for joint exercises in the Yellow Sea – which lies between the Korean peninsula and China – earlier this year. They were moved to the Sea of Japan, on the east coast.

Crowley sought yesterday to play down the artillery barrage, characterising it as an isolated incident, but South Korean media claimed that Kim Jong-il and his son and heir, Kim Jong-un, had visited the artillery base used to shell Yeonpyeong hours before the attack. The reports are impossible to verify and North Korea released an undated photo of Kim visiting a factory shortly after the attack.

In Seoul, the Chosun Ilbo newspaper quoted an unnamed member of the National Assembly's defence committee as saying that South Korea's military was "focusing on the possibility of Kim Jong-il and his son had approved the shelling of Yeonpyeong".

True or not, the claims will fuel anger in Seoul.

"Let me say a word about those bastards at the Blue House [presidential palace] who advised the president to say the situation should be managed to avoid a full-blown war," the Korea Joongang Daily quoted a ruling party representative, Hong Sa-duk, as saying. "They must all be fired for advising the president to have such a weak response."