North Korea has recently deployed dozens of attack helicopters near its disputed sea border with South Korea, media reports said Tuesday, as both sides strengthen their military presence in the area.

The North has posted some 50 helicopters to Taetan and Nuchon air bases near the Yellow Sea, Yonhap news agency and other media reported, citing unidentified Seoul officials.

They include upgraded versions of the North's Mi-2 helicopters as well as Mi-4 and Mi-8 models imported from the Soviet Union decades ago, it said, adding some were armed with machine guns and rockets.

"These choppers are being used for ground attacks and high-speed mobility forces exercises," Yonhap quoted an official as saying.

Helicopters from the two bases could reach the South's frontline island of Baengnyeong within two or three minutes, Chosun Ilbo newspaper said.

Seoul's defence ministry declined to comment.

The border was the scene of deadly naval clashes in 1999, 2002 and 2009. In November 2010 the North shelled one of the South's five border islands, Yeonpyeong, and killed four South Koreans including two civilians.

The South has since strengthened manpower and weaponry on the islands to forestall any fresh attack. Seoul said last month it planned to build an advance naval base on Baengnyeong.