Somali pirates freed a Japanese ship held hostage for months, but a maritime watchdog on Friday warned that pirates are stepping up their attacks as weather improves in the Indian Ocean and they look to rake in more ransoms.

U.S. Navy and Russian warships arrested 26 suspected pirates off Somalia in separate operations this week, while a maritime watchdog warned on Friday that attacks are stepping up as weather improves and pirates look to replenish their haul after releasing ships hijacked for ransom.

The latest arrests came Thursday, when an American helicopter from the USS Vella Gulf fired warning shots at gunmen in two skiffs that had opened fire and tried to board the Indian-flagged vessel Premdivya.

U.S. forces searched the skiff and found weapons including rocket-propelled grenades, then took nine suspected pirates aboard the American ship, the Bahrain-based 5th Fleet said.

On Wednesday, the same American ship detained seven other suspected pirates — the Navy's first arrests since it established an anti-piracy task force this year. The suspects, armed with AK-47s and rocket-propelled grenades, had tried to board the Marshall Islands-flagged vessel Polaris using a ladder from their skiff.

The pirates were transferred via helicopter to the USNS Lewis and Clark on Thursday, according to Lt. Nathan Christensen, a 5th Fleet spokesman. Pentagon spokesman Bryan Whitman said they could be handed over to Kenya, after the United States last month agreed to hand pirate suspects to the east African nation.

Associated Press Television News footage from aboard the Lewis and Clark showed some of the men, handcuffed and wearing leg shackles and white jumpsuits, being escorted from helicopters onto the ship.

They were given a meal, a blanket, a towel and a bar of soap, but they were not allowed to talk to each other. U.S. forces assisted by a translator were trying to get information from the men, such as their ages and nationalities. The men were then taken to a holding area surrounded by razor wire and guarded.

Separately, the Russian navy said Friday its nuclear-powered heavy missile cruiser Peter The Great detained 10 Somali pirates closing in on an Iranian-flagged fishing trawler. Russian military prosecutors were questioning the men, who were caught on Thursday with rifles, grenade-launchers, illegal narcotics and a large sum of money, the navy said.

Piracy off Somalia, which has not had a functioning government since 1991, reached record levels last year. Somali pirates, seeking multimillion-dollar ransoms, launched 111 attacks and seized 42 vessels last year, mostly in the Gulf of Aden, with attacks peaking between September and November.

Somali piracy accounted for the bulk of the 49 vessels hijacked and 889 crewmembers taken hostage around the world in 2008 — the highest worldwide figures since the London-based International Maritime Bureau began keeping records in 1991.

The increased danger led the United States, India, Britain, China, Russia, Saudi Arabia, South Korea and other countries to send warships to the area to protect commercial vessels and more quickly rush to their aid.

Noel Choong, chief of the bureau's reporting center in Kuala Lumpur, said six ships were attacked on Wednesday and Thursday alone. "We haven't seen such an increase in attacks for some time," said Choong.

Since the beginning of January, 22 vessels had been attacked, and three were hijacked. Choong said favorable weather made it easier for the smaller pirate boats to ambush ships. He also said seven ships have been released over the past month, likely pushing pirates to try to replenish their stocks.

In the latest release, pirates freed a Japanese-owned cargo ship and its 23 crew members after nearly three months in captivity, a diplomat based in Nairobi, Kenya, said Friday.

The MV Chemstar Venus, owned by a Japanese shipping company and registered in Panama, was seized by armed Somali gunmen on Nov. 15 in the Gulf of Aden. On board were five South Koreans and 18 Filipinos. The diplomat, who asked not to be named because he was not authorized to speak to the media, said the ship carrying unidentified chemicals was released Thursday night. There was no immediate word if a ransom was paid.

In the aftermath of what was one of the most dramatic pirate heists, Ukrainian sailors returned home after four months in captivity after the seizure of their cargo ship, MV Faina, which was loaded with tanks and heavy weapons. The sailors stepped off a plane in Kiev, tanned but exhausted-looking for a tearful reunion with their families on the tarmac.

Faina's ordeal began in September, when scores of heavily armed Somali pirates swarmed onboard as it carried 33 Soviet-designed tanks and crates of small arms headed to Kenya. The pirates released the vessel last week, reportedly after receiving a $3.2 million ransom.

The Faina, with its cargo, docked on Thursday at the Kenyan port of Mombasa. Foreign governments had feared the Faina's weapons might fall into the hands of Somali insurgents, who the U.S. State Department says are linked to Al Qaeda.