US NAVY commandos sank a South Korean warship in March in order to blame North Korea, raise tensions and convince Japan to keep US forces in Okinawa, Fidel Castro has claimed.

The former Cuban president, who based his information on press reports, blasted Washington's "cynicism" and "lack of scruples" in the incident in a newspaper editorial.

According to Mr Castro, US Navy SEALS torpedoed South Korea's Cheonan corvette in a bid by Washington to sway Japan to allow a US military base to remain on Okinawa island.

Mr Castro, 83 and convalescing away from government since 2006, writes regular columns for Cuban media commenting on world events.

A multinational investigation last month concluded that a North Korean submarine torpedoed the South Korean warship on March 26, killing 46 people on board.

Yukio Hatoyama resigned as Japan's Prime Minister this week amid a row over an unpopular US Marine Corps airbase on Okinawa, having failed to fulfill a campaign promise to close the military facility.

Mr Castro said rising tensions on the Korean peninsula were among the causes for Hatoyama's resignation.

"Political leaders and world opinion have proof of the cynicism and lack of scruples that characterises US imperial policy," he wrote.

"Thus, in a surprisingly easy manner, the United States managed to solve an important problem: to liquidate the national unity government of Yukio Hatoyama's Democratic Party of Japan."

He noted that Washington's alleged hand in the Cheonan ship came at a high price, having "deeply offended its South Korea allies".

The former Cuban leader also hailed fellow communist Kim Jong Il, who has rejected any responsibility in the incident, and highlighted China's muted response to the conflict.

The Asian giant has resisted pressure to condemn North Korea and has called for efforts to ease regional tensions.

Originally published as Castro claims US sank South Korean ship