Seoul, South Korea (CNN) -- U.S. Secretary of State Hillary Clinton pledged Washington's support for South Korea and called on North Korea to end belligerent actions after meetings with President Lee Myung-bak and Foreign Minister Yu Myung-hwan on Wednesday.
"We will stand with you in this difficult hour and we will stand with you always," Clinton said at a news conference with Yu.
We call on North Korea to halt its provocations and its policy of threats and belligerence toward its neighbors and take steps now to fulfill its denuclearization commitments and comply with international law."
The meetings come at a time of high tensions on the Korean peninsula after Seoul blamed North Korea for the sinking in March of the South Korean warship Cheonan.
An official South Korean report has accused the communist North of firing a torpedo at the ship, killing 46 sailors.
"This was an unacceptable provocation by North Korea and the international community has a responsibility and a duty to respond," according to Clinton, who promised to push for possible sanctions in the U.N. Security Council over the incident. Clinton met this week with Chinese leaders in Beijing, but has so far been unable to garner their backing for action against Pyongyang.
Meanwhile, a North Korean military official accused the South of intruding into North Korean waters in the Yellow Sea from May 14 to May 24, the Yonhap news agency reported Tuesday.
"This is a deliberate provocation aimed to spark off another military conflict in the West Sea of Korea and thus push to a war phase the present north-south relations," the official said in a statement, according to Yonhap.
Lee has already announced that South Korea was suspending trade with North Korea, closing its waters to the North's ships and adopting a newly aggressive military posture toward its neighbor.
"The U.S. and South Korean militaries have announced plans for joint exercises and we will explore further enhancements to our posture on the peninsula to ensure readiness and to deter future attacks," Clinton said. "The United States is also reviewing additional options ... to hold North Korea and its leaders accountable."
While in China on Monday, Clinton said the United States supports the finding on the Cheonan's sinking and said the United States' "support for South Korea's defense is unequivocal."
"We endorse President Lee's call on North Korea to come forward with the facts regarding this act of aggression and, above all, stop its belligerence and threatening behavior," Clinton said Monday.
U.S. President Barack Obama has directed military commanders to work with South Korean troops "to ensure readiness and to deter future aggression" from North Korea.