SEOUL, June 1, 2009 (AFP) - North Korea is stepping up naval landing exercises amid growing tensions with South Korea, which is not ruling out an attack on one of its islands, South Korea's Yonhap news agency reported Monday.

The North has conducted shooting drills or used high-speed boats for landing exercises and has banned ships from some areas of the Yellow Sea until the end of July, Yonhap reported.

"North Korea has increased exercises using high-speed amphibious boats. We are not ruling out the possibility that North Korea may try to raid one of the islands," it quoted an unidentified defence source as saying.

Tensions have been running high for the past week after North Korea tested a nuclear bomb for the second time, launched a series of short-range missiles and warned that it could launch an attack on the South.

South Korea is on heightened alert for a possible repeat of the deadly naval clashes seen in 1999 and 2002 near the tense maritime border area.

A South Korean defence ministry spokesman told AFP on Monday that North Korea had stepped up military activities following its threat of a possible attack on the South.

"North Korean troops have been conducting more drills, especially along the west coast," the spokesman said.

He declined to confirm a Yonhap report that the North had doubled the storage of ammunition for artillery units and patrol boats along the west coast.

Pyongyang has warned it would take "additional self-defence measures" in response to any tougher international sanctions and South Korea said Monday that the North may be preparing another long-range missile test.