The US government has warned ships sailing off Yemen's coast that they are susceptible to attacks by al-Qaeda.
The US Office of Naval Intelligence said on its website on Monday that ships in the Red Sea, the Bab al-Mandab Strait between Yemen and Djibouti, and the Gulf of Aden along Yemen's coast, were at the greatest risk.
"Information suggests that al-Qaeda remains interested in maritime attacks in the Bab al-Mandab Strait, Red Sea, and the Gulf of Aden along the coast of Yemen," the office said in a statement, citing an advisory by the US department of transportation.
"Although it is unclear how they would proceed, it may be similar in nature to the attacks against the USS Cole in October 2000 and the M/V Limburg in October 2002 where a small to mid-size boat laden with explosives was detonated," it added.
Yemen, at the forefront of Western security concerns since a failed December attack on a US-bound plane, boosted security on its coast earlier this year to prevent fighters reaching its shores from nearby Somalia to reinforce al-Qaeda in Yemen.
Al-Qaeda's Yemen-based arm claimed responsibility for the failed December plane attempt.
'Sophisticated attacks'
Western allies and neighbouring oil exporter Saudi Arabia fear al-Qaeda is exploiting instability on several fronts in impoverished Yemen to recruit and train fighters for attacks in the region and beyond.
The transportation department statement said more sophisticated methods of attack by al-Qaeda in the waters near Yemen could include missiles or projectiles.
"All vessels transiting the waters in the vicinity of Yemen are urged to operate at a heightened state of readiness," the statement said, adding that vessels were at greater risk in areas of limited manoeuvrability or while anchored or at port.
The US navy warship USS Cole was hit by a suicide attack in the Yemeni port of Aden in 2000 leaving 17 navy personnel dead and 39 wounded. Al-Qaeda claimed responsibility for the attack.
Two years later an al-Qaeda attack damaged the French supertanker Limburg in the Gulf of Aden.