The attacks, one on a United Arab Emirates (UAE) general cargo ship on Sunday and another launched from a so-called "mother ship" last Friday, come only days after a US Navy warship captured a band of suspected pirates with a cache of arms.
Jayant Abhyankar, deputy director of the International Maritime Bureau (IMB), told Reuters on Wednesday: "With these attacks we now have a grand total of five ships and around 100 mariners being held hostage in Somalia, it was as many as 200, and some have been held for four months."
The IMB said gunmen stormed the UAE-owned Al-Manara 150 nautical miles off the east coast of Somalia, threatened the crew, and are now demanding a ransom for their release.
One industry newspaper on Wednesday reported that the crew were low on drinking water and supplies.
In a second attack, the IMB said pirates in two speed boats, chased and fired machine-guns at a dry-bulk ship off Somalia's east coast before giving up on an attempted boarding.
The IMB said the speed boats were launched from a "mother ship" similar to one that launched attacks in the Indian Ocean last November.
The bureau said there had been 38 attacks since last March, including against oil tankers and cruise ships.
Merchant shipping shaken
The wave of attacks has badly shaken merchant shipping, which relies heavily on key international trade routes that snake down Somalia's coastline, Africa's longest.
Only at the weekend the US Navy reported it had pursued and caught a suspected pirate ship after receiving a report of an attempted attack.
According to the IMB, the US Navy is interrogating 10 Somali men and 16 Indians found on board. The Indians are thought to be crew from a vessel hijacked near Mogadishu and used to attack a number of other vessels.
The US Navy's Fifth Fleet based in the Gulf has issued a warning to merchant shipping to stay at least 200 nautical miles off Somalia's coast.
Somalia has been without a central government since 1991, when rival warlords ousted Mohamed Siad Barre.
In November, the Somali government signed a two-year deal worth $50 million with a US marine security firm to try to end piracy.