The unusual move comes as South Korean President Lee Myung-bak visits key ally the United States this week to discuss trade ties and how to move ahead with diplomatic efforts to end the North’s nuclear arms ambitions.

The news agency quoted a South Korean government official as saying that the North has moved the planes and short-range missiles to bases and installations near the disputed sea border off the peninsula’s west coast.

Calls by Reuters to the South’s Defense Ministry and the office of the Joint Chiefs of Staff went unanswered. Other military officials could not be reached immediately for comment.

South Korea has put in round-the-clock surveillance of the North’s military movements along the west coast after two deadly attacks last year killed 50 people, driving tensions on the peninsula to the highest level in decades.

A torpedo attack against a South Korean navy ship in March last year killed 46 sailors, but the North has denied responsibility. An artillery attack on a Southern island in November killed four more people, including two civilians.

Yonhap said the North appears to have moved the same type of artillery used in the November attack closer to the South.