Merchant vessels have been among targets for Somali pirates
Four more vessels have been attacked by armed bandits off Somalia, as the UN Security Council said foreign forces could pursue the pirates on land.
Maritime officials say a Chinese ship's crew held off one group of attackers until a naval patrol arrived.
But an Indonesian tugboat, Turkish cargo ship and a private yacht were successfully boarded and are thought to remain in pirates' hands.
The attacks persist despite increased US, European and Indian naval patrols.
The BBC's Peter Greste in Nairobi says the latest attacks appear to be a calculated jab at UN attempts to clamp down on piracy.
He says there have been 42 successful hijackings in the area this year.
Fourteen foreign ships and their crew of over 200 are still being held, our correspondent adds.
'All necessary measures'
On Tuesday the UN Security Council unanimously approved a resolution allowing foreign military forces to pursue pirates on land in Somalia.
A surge in pirate attacks has proved costly to international shipping
Permission will be needed from the transitional Somali government for any incursions. Countries already have powers to enter Somali waters to pursue pirates.
China told the meeting in New York it was considering sending a naval force into the region.
The US-drafted resolution was the fourth approved by the Security Council since June to combat piracy in the region.
It gives authority for one year for countries to use "all necessary measures" by land or air to stop anyone using Somali territory for piracy.
Indonesia was among critics of the plan as it feared the precedent it could set for chasing pirates on land, but it voted in favour.
Russia's Foreign Minister Sergey Lavrov says the problem of piracy off the coast of Somalia cannot be solved by the use of force alone.
"We should not forget that piracy and armed robbery at sea in this region are just the tip of the iceberg of the problems Somalia is facing today," he told Russian television.
"We stand for active efforts to stabilise social, economic and political situation in Somalia which will make it possible to undermine the material base of piracy."
On Wednesday, Chinese sailors backed up by a coalition warship and helicopters foiled a pirate attack in the Gulf of Aden, a maritime watchdog said.
The pirates boarded the ship but crew members locked themselves in their cabins to prevent the bandits entering and called for help, said the International Maritime Bureau. The attackers reportedly fled.
Three other vessels were seized on Tuesday.
Pirates seized a Malaysian tugboat owned by French oil firm Total on its way home through the Gulf of Aden from the Middle East.
A Turkish cargo ship, MV Bosphorus Prodigy, owned by Isko Marine Company, was also captured. And a private yacht was seized.