South Korea staged a live-fire military drill Friday near its disputed Yellow Sea border with North Korea, despite warnings from Pyongyang ahead of a sensitive anniversary.

The drill was held on two front-line islands -- one of which was shelled by North Korea on November 23, 2010, killing four people and briefly triggering fears of a full-scale conflict.

North Korea has sent a rare message to the South Korean president's office, demanding an end to anti-Pyongyang leaflet launches into the North, officials said Tuesday.

North Korea said Tuesday it would strengthen its nuclear deterrent following President Barack Obama's "dangerous" Asian tour, and would not rule out another atomic test.

There are concerns the North is preparing to conduct its fourth atomic detonation, with recent satellite images showing stepped-up activity at its main nuclear test site.

North Korea test-fired four short-range missiles into the sea Thursday, Seoul's defense ministry said, in an apparent show of force to coincide with South Korea-U.S. joint military exercises.

A ministry spokesman told Agence France Presse the missiles, with an estimated range of 200 kilometers (125 miles), were fired off the east coast of North Korea.

The execution of the North Korean leader's uncle is "an ominous sign" raising concerns about instability in a nation pursuing a nuclear arms drive, U.S. Secretary of State John Kerry warned Sunday.

Pyongyang on Thursday executed the uncle of North Korean leader Kim Jong-Un charging him with corruption and plotting to overthrow the state.

North Korea on Monday warned of "merciless firing" against the South if it goes ahead with a reported plan to develop shells to carry anti-Pyongyang leaflets across the border.

The South's Joong-Ang Ilbo newspaper reported last week that South Korean troops were developing non-explosive hollow shells capable of carrying such leaflets deep into North Korean territory. There has been no official confirmation.

North and South Korea reached an agreement Wednesday to re-open the Kaesong joint industrial zone -- closed by Pyongyang in April at the height of soaring military tensions.

The five-point agreement that came out of a seventh round of talks committed both sides to making "active efforts" to resuming normal operations as soon as possible after inspecting the shuttered plants in Kaesong.

A top North Korean envoy said Friday that U.S. hostility could lead to war at any time, but reaffirmed a government offer of talks with Washington that could include the nuclear weapons issue.

At a rare but typically combative news conference, the isolated state's U.N. ambassador Sin Son-Ho accused the United States of driving up tensions and appealed for an end to U.N. and U.S. sanctions against Pyongyang.

The United States on Sunday welcomed North Korea's proposal for high-level negotiations but said it must first curb its nuclear program and would not be able to talk its way out of global sanctions.

White House Chief of Staff Denis McDonough said negotiations have always been the administration's "preferred outcome" but that such talks would have to be "real" and "based on them living up to their obligations."

North Korea on Thursday blamed South Korean arrogance and deceit for the collapse of planned talks between the two rivals and warned that prospects for any future dialogue had been severely damaged.

The two Koreas had initially agreed to hold their first high-level talks in six years in Seoul on Wednesday and Thursday this week, but they were called off at the last minute following a dispute over protocol.