As a hostage standoff with Somali pirates continued Thursday, a top U.S. army leader said the country will boost its military presence near the Horn of Africa within two days.

"We want to ensure that we have all the capability that might be needed over the course of the coming days," Gen. David Petraeus, head of the U.S. Central Command said Thursday in West Palm Beach, Fla. He did not give specifics.

Somali pirates tried to hijack the U.S. cargo ship Maersk Alabama in waters near the Horn of Africa on Wednesday, but the crew thwarted the attempt.

The armed pirates escaped on one of the ship's lifeboats, taking the Maersk captain hostage.

In this family photo released on Wednesday, Capt. Richard Phillips of Underhill, Vt., is seen. Phillips is the captain of the U.S.-flagged cargo ship Maersk Alabama which was hijacked Wednesday by Somali pirates off the Horn of Africa. (Associated Press)

Meanwhile, FBI hostage negotiators have joined the efforts to secure the captain's release.

Pirates hijacked the 17,500-tonne Maersk Alabama early Wednesday about 640 kilometres east of the Somalian capital of Mogadishu, but the 20 U.S. crew members later retook control of the ship. The crew had also captured one of the four pirates that hijacked the ship, and held him for 12 hours.

A later attempt to swap the captive pirate for the ship's chief officer went awry — the pirate was freed while the captain, Richard Phillips, wasn't. Another officer aboard the Alabama, Ken Quinn, told CNN that the pirates had reneged on an agreement to release Phillips in exchange for one of their own.

The pirates instead made off with Phillips in a small lifeboat.

That lifeboat is now out of fuel and is floating near the Alabama in Somali waters, said Kevin Speers, a spokesman for the ship's owner, Maersk.

"The boat is dead in the water," he told the Associated Press. "It's floating near the Alabama. It's my understanding that it's floating freely."

Phillips volunteered himself

Phillips surrendered himself to the pirates to secure the safety of the crew, said Gina Coggio, the half-sister of Phillips' wife, Andrea Coggio.

"That is what he would do. It's just who he is and his response as a captain," she said from Phillips's home in Vermont.

The USS Bainbridge, an American destroyer dispatched to waters off the Horn of Africa, has begun negotiations with the pirates, said the CBC's David McGuffin, reporting from Nairobi.

It is not known what the pirates are seeking, and Speers said the pirates haven't contacted Maersk with any demands yet.

"So we're now in a situation where it is effectively a bit of a standoff," said McGuffin.

"Negotiations are going on, and we're really just waiting to hear word from the U.S. Navy on exactly what the next step is."

Ship heading to original destination

FBI spokesman Richard Kolko described the bureau's hostage negotiating team as "fully engaged" with the military in strategizing ways to retrieve the ship's captain.

The Alabama, meanwhile, has resumed its journey to its original destination, the Kenyan port of Mombasa, said the father of one of those aboard the ship.

Capt. Joseph Murphy, a professor at the Massachusetts Maritime Academy whose son, Shane Murphy, is second in command, said 18 armed guards are aboard the vessel.

Joseph Murphy said he was told about the development by company officials who are briefing families and estimates the ship will arrive in Kenya on Saturday. The Maersk Alabama, loaded with relief aid, had been en route to Mombasa when pirates attacked it.

The ship is the sixth to be seized within a week. Many pirates, who have traditionally had success attacking ships in the Gulf of Aden, one of the world's busiest shipping lanes, have shifted their operations down the Somali coastline to escape naval warship patrols, which had some success in preventing attacks last year.

A dozen warships from countries including Britain, France, Germany, Iran and the United States now dot the notorious waters in the gulf in an attempt to deter pirate attacks.

With files from the Associated Press