AllAfrica.com reported a total of more than 60 crew members were taken hostage in the maritime hijackings Monday and Tuesday.
The latest ships hijacked by pirates were the Irene EM, a Greek-owned bulk carrier; the MV Sea Horse, a Lebanese-owned cargo ship; and two Egyptian fishing vessels, the BBC reported.
The NATO Shipping Center also reported a number of unsuccessful attacks on shipping in recent days, AllAfrica.com said.
Fox News reported the Liberty Sun, with a crew of about 20 U.S. citizens, alerted the U.S. Coast Guard it was under attack from rocket-propelled grenades and automatic weapons Tuesday. But by the time the Navy's USS Bainbridge several hours later, the buccaneers had left.
"We are grateful and pleased that no one was injured, and the crew and the ship are safe," Liberty Maritime Corp. of Lake Success, N.Y., said in a statement. "We commend the entire crew for its professionalism and poise under fire."
The Liberty Sun, which had traveled from Houston, resumed its voyage to Mombassa, Kenya, with food for famine-stricken African countries.
Meanwhile, the rescued captain of a U.S. ship was en route Tuesday to Mombasa, Kenya, to join his crew, CNN reported.
Richard Phillips was freed Sunday when U.S. Navy snipers killed three pirates aboard a lifeboat where they held Phillips hostage after their attempt to hijack his ship failed. A fourth pirate was being held by U.S. authorities for prosecution, CNN said.
Phillips was to join his crew at a beach resort in Kenya before returning home to the United States.