The US is sending an aircraft carrier to boost its patrols in the waters off Yemen, amid suspicions that an Iranian naval convoy in the area is carrying arms intended for Yemeni Houthi rebels.
The US has warned Iran against arming the rebels – but a US spokesman said the ships would not stop the convoy.
The US navy, which already has seven boats in the area, says its deployment aims to secure shipping lanes.
Yemen is gripped by conflict between the rebels and government supporters.
A rebel advance drove out the president, Abdrabbuh Mansour Hadi, last month. His supporters, backed by a Saudi-led air campaign, have been battling the rebels in and around the southern port city of Aden.
The US is not taking part in the air strikes, but it has backed the Saudi-led coalition by boosting arms deliveries and intelligence sharing.
The USS Theodore Roosevelt, an aircraft carrier, and a guided-missile carrier, the USS Normandy, are travelling from the Gulf to join other vessels in the Arabian Sea off Yemen, US officials said.
The other US boats in the area include two destroyers, two mine-sweepers and three amphibious ships, Reuters news agency reports.
A US spokesman denied the ships would intercept the Iranian convoy, which is believed to be approaching Yemen.
However, a US official, interviewed by AFP news agency, expressed concern that the convoy may be carrying weapons for the Houthis, while admitting that it was too early to speculate.
“We believe these vessels may have arms and equipment on board. If they are delivered to Yemen, it will further destabilise” the country, the official was quoted as saying.
Sunni-ruled Saudi Arabia has accused its regional rival, Shia Iran, of aiding the Houthis, who are from the Zaidi Shia sect. Iran denies arming them.
A UN resolution last week imposed an arms embargo on the rebels and their allies.
The UN says 150,000 people have been displaced by the latest fighting in Yemen, and some 12 million are short of food.
It has also said 731 people had been killed and 2,754 injured – many of them civilians – in three weeks between March and April.
The numbers are likely to be an underestimate, it has warned.