The military strategy to encircle the seized vessel came as another official said the pirates had reduced their ransom demand from $35 million to $20 million.

Navy Lt. Nathan Christensen told The New York Times "several destroyers and missile cruisers" had joined a U.S. destroyer already following the Faina, effectively surrounding the pirates. The Navy's plan for dealing with the maritime hijackers should they refuse to surrender was not disclosed, the U.S. newspaper said.

"Our intent is for the ship not to offload any of its cargo," he said.

The Ukrainian ship, which is carrying 33 Soviet-made T-72 tanks and other ordnance, and its 21-member crew were hijacked last Thursday near Mombasa, Kenya.

Andrew Mwangura of Kenya's Seafarers Assistance Program told CNN the organization's sources in Somalia reported the reduced ransom amount.

The Kenyan government said Sunday it hadn't been contacted by the hijackers. Rashid Abdi, an International Crisis Group analyst, said the pirates have been releasing their ransom demands through media interviews.

The pirates reportedly have warned against attacks by U.S. or Russian forces.

The American military and others are concerned the tanks and other weapons could reach Islamist insurgents in Somalia.