The US and South Korea intend to work together on two major military exercises off the Korean Peninsula.
The White House said on Monday that Barack Obama, the US president, has ordered his military commanders to co-ordinate closely with their South Korea counterparts "to ensure readiness and to deter future aggression" by North Korea.
Bryan Whitman, a Pentagon spokesman, said the joint exercises will be conducted in the "near future" and will test the nations' ability to defeat submarines and to monitor and prevent illicit activities.
Kim Tae-Young, South Korea's defence minister, said the two countries' navies will soon stage an anti-submarine drill off the west coast and that Seoul will resume the border broadcasts halted six years ago.
The display of force comes after last week's finding by a multinational investigation that North Korea torpedoed the Cheonan, a South Korean warship, that left at least 46 sailors dead on March 26.
It was the nation's worst military disaster since the 1950-53 Korean war.
Earlier on Monday, South Korea's president vowed that North Korea will pay the price for what he called its "brutality" in the sinking of the Cheonan, a 1,200-tonne corvette.
Call for apology
In a televised address, Lee Myung-bak said: "I solemnly urge the authorities of North Korea ... to apologise immediately to the Republic of Korea (South Korea) and the international community".
He ordered a ban on all trade, investment and visits with North Korea and stopped their commercial shipping using a cheaper route through its waters.
Lee also vowed to take North Korea to the UN Security Council over the sinking.
For his part, Ban Ki-moon, the UN secretary-general, who is a former South Korean foreign minister, said he was confident the Security Council would take measures "appropriate to the gravity of the situation".
He said he hoped "the council's prompt action will also contribute to the early resumption of the six-party talks to address nuclear issues and other outstanding concerns".
The stalled six-party aid-for-disarmament talks included North and South Korea, China, Russia, the US and Japan.
North Korea is currently under UN sanctions for testing nuclear devices in 2006 and 2009.
The White House called South Korea's measures to punish the North entirely appropriate and asked Pyongyang to stop its "belligerent and threatening behaviour".
Hilary Clinton, the US secretary of state, avoided answering a question on whether the US would support additional UN sanctions.
However, while opening high-level US-China talks in Beijing, Clinton said North Korea must be held to account for the incident.
China is North Korea's only major political and economic backer, and Beijing has avoided firm public comment on the sinking, instead expressing sympathy for the South Koreans killed and urging all sides to show restraint.
Al Jazeera' China correspondent, Melissa Chan, said China had to perform a juggling act, weighing its relationship with North Korea against its ties with the US.
The fact that China has been so cautious shows that it is very worried that the situation could lead to all out war, she said.
Leonid Petrov, a North Korean specialist and a lecturer in Korean studies at the University of Sydney, told Al Jazeera that relations between the North and South had been deteriorating for some time.
"The inter-Korean relationship is destroyed now, co-operation has crumbled ... the result was the sinking of the Cheonan, and I can see it's a very logical result of this collapse in communication and inter-Korea relations," he said.
"Such accidents could have been easily prevented by diplomatic means - this is something President Lee jeopardised by scrapping the 'sunshine policy' [of rapprochement towards the North]."
The two Koreas are still technically at war because their 1950-53 conflict ended with an armistice, not a peace treaty.