Somali pirates left two boats they had hijacked in the waters off the Horn of Africa, and the newly liberated vessels -- and their crew of 24 -- were under U.S. Navy escort on Sunday, the American military said.

A U.S. Navy ship and helicopter were guiding the Tanzanian-flagged boats Mavuno 1 and 2 further out to sea, where naval personnel will later board the vessels and treat crew members, said Cmdr. Lydia Robertson of the U.S. Fifth Fleet in Bahrain. The Navy is in radio contact with pirates aboard three other ships in the region, encouraging them also to leave those ships and sail back to Somalia, she told The Associated Press.

"We're very happy with this development and hope it happens with the other ships off the coast," Robertson said. "We're very happy for the crew and their families."

Robertson said the pirates boarded skiffs after they left the hijacked ships, and headed back to Somalia. No shots were fired during the incident, she said. She gave no more details.

The U.S. has now intervened four times in one week to help ships hijacked by Somali pirates. Sailors boarded a North Korean ship to give medical assistance to crew members who overpowered their hijackers, and a Naval vessel fired on pirate skiffs tied to a Japanese-owned ship.

Robertson said that ship was still under control of pirates, although the U.S. Navy was still working to free that ship from pirates. There were no details on the other two seized ships. Hijackings in the vast stretch of water frequently go unreported.

In South Korea, the Foreign Ministry said 24 sailors onboard the two Korean-owned ships seized May 15 off Somalia were safe. The ministry said the ships were being escorted to a port in Yemen by a U.S. Navy warship at the request of the South Korean government, the ministry said in a statement. The two dozen sailors were comprised 10 Chinese, four South Koreans, three Vietnamese, three Indians and four Indonesians.

South Korean media have reported that the Somali pirates were demanding between $700,000 and $1 million in ransom. Robertson had no comment on ransom demands, deferring to the shipping company.

Somalia lies close to crucial shipping routes connecting the Red Sea with the Indian Ocean, where valuable cargo and carriers must pass.

Somalia has had no effective government since 1991, when warlords ousted a dictatorship and then turned on each other. The country's 1,880-mile coastline makes it difficult to prevent attacks.

Last year, another South Korean fishing vessel was captured off Somalia and released three months later after a ransom of more than $800,000 was paid.