Pirates have captured four more ships and taken more than 60 crew members hostage in the Gulf of Aden in the last two days, following the death of three pirates in a standoff with the United States Navy.

Irene EM, a Greek-owned bulk carrier, was captured overnight Monday, according to the shipping news website Lloyds Register-Fairplay. The
22-man crew is believed to be unharmed.

In addition, the BBC is reporting that the MV Sea Horse, a Lebanese-owned cargo ship, was captured Tuesday. On Monday, pirates seized two Egyptian fishing vessels, according to the BBC.

The Nato Shipping Centre also reports a number of unsuccessful attacks on shipping in recent days.

On Sunday, U.S. Navy Seals shot dead three pirates in order to free Richard Phillips, the captain of a American-flagged vessel who was taken hostage. Earlier in the week, French armed forces stormed a yacht, resulting in the deaths of two pirates and one hostage.

Pirates have vowed revenge on the U.S. and France. One pirate told AllAfrica publishing partner Shabelle that they would kill any captured French or American citizens.

On Monday, U.S. President Barack Obama vowed to crack down on piracy. "I want to be very clear that we are resolved to halt the rise of piracy in that region and to achieve that goal, we're going to have to continue to work with our partners to prevent future attacks," Obama said in a speech in Washington DC.

The piracy reporting centre of the International Maritime Bureau has warned shipping off the Somali coast that intelligence sources have
revealed that "that there are a number of suspicious vessels in the Gulf of Aden believed to be pirate mother vessels looking to attack ships with the intent to hijack."

The centre's website carries photos of Russian-made trawlers suspected to be mother ships.

"All vessels... are advised to take additional precautionary measures and maintain strict 24-hour visual and radar anti-piracy watch using all
available means." the centre said. "Watch-keeping crews should look out for small suspicious boats...

"Early sightings/detection and accurate assessment will allow [a ship's]  master to increase speed and take evasive manoeuvres to escape
from pirates and at the same time request assistance..."