US President Barack Obama spoke to his South Korean counterpart Lee Myung-bak, renewing Washington's "unshakeable support" for the country in the wake of a North Korean missile attack that killed two marines.
Mr Obama told President Lee that the thoughts and prayers of the American people were with the loved ones of those who were killed or wounded in the attack.
The President said North Korea must stop its provocative actions, which will only lead to its further isolation, and fully abide by the terms of Armistice Agreement and its obligations under international law, according to the White House.
Two South Korean marines were killed and five were seriously injured when North Korea allegedly shelled a border island in what Seoul said was a "pre-planned" attack on Tuesday.
Mr Obama also had a situation room meeting with his top national security advisers about the situation on the Korean peninsula.
The President was briefed by national security advisor Tom Donilon, secretaries Hillary Clinton and Robert Gates, United States ambassador to the United Nations Susan Rice, Admiral Mike Mullen, and General Walter Sharp, the Commander of US forces Korea.
"The President reiterated the unshakeable support of the United States for our ally, the Republic of Korea, and discussed ways to advance peace and security on the Korean peninsula going forward," the White House said.
"The two Presidents agreed to hold combined military exercises and enhanced training in the days ahead to continue the close security cooperation between our two countries, and to underscore the strength of our Alliance and commitment to peace and security in the region," it said.
The White House said the United States remains firmly and fully committed to the defence of its ally. "The United States will work with the international community to strongly condemn this outrageous action by North Korea, and to advance peace and security in the region," it said.
Briefing reporters earlier in the day, Deputy White House press secretary Bill Burton described Mr Obama has being "outraged" by North Korean actions. He said the United States "stands shoulder to shoulder" with South Korea and that it is fully committed to South Korea's defence.
"It's an outrageous act. The President thinks that North Korea is not living up to their obligations and they ought to live up to the obligations that are signed in the armistice agreement and international law," Burton said.
He said North Korea needs to live up to its international obligations and make real progress in ending its "illegal" nuclear programme.
"North Korea has a pattern of doing things that are provocative. This is a particularly outrageous act, and we're going to be doing everything that we need to do in order to make sure that we're defending our ally in South Korea and that there's security and stability in the region," he said.