Kenya warplanes targeted the Shebab-held Somali port city of Kismayo on Sunday as troops advanced on the insurgents and the US warned of an imminent threat of attack on foreigners in Kenya.

Nairobi sent soldiers across the border a week ago to hunt the Al-Qaeda-linked Shebab fighters it blames for the abductions on its soil of a British tourist, a disabled French woman and two Spanish aid workers.

"The aircrafts targeted Shebab positions including a military base and the seaport but we dont know if there were any casualties," said resident Abdikarim Maolim.

"Residents are shocked and the city is tense," he added.

Ahmed Yasin, another witness said: "I saw the two planes flying at low level but high speed and after five minutes they dropped heavy bombs that rocked the city."

Shebab official Sheik Abddala Abu-Hassan said by phone that the raids had caused civilian victims but left their fighters unhurt.

This weekend the Kenyan army said troops were advancing in a three-pronged movement towards the city, about 250 kilometres (155 miles) from the Kenyan border.

The military has not said how many of its troops are deployed, but analysts estimate the number at between 2,000 and 3,000.

The radical Islamist Shebab, who deny kidnapping foreigners, has repeatedly warned of bloody retaliation against Kenya.

In response to the threats, officials have beefed up security in Nairobi's central business district and the US embassy there warned Americans of an "imminent threat" of attacks possibly targeting foreigners.

Secretary of State Hillary Clinton said in a US television interview: "We've been getting threats from Shebab against Americans and Westerners.

"It's a very dangerous, uncertain situation, and we want to be sure that whatever information we have, we immediately present to Americans who live, work or may be visiting in Kenya," she told ABC television.

The embassy statement said there was "credible information of an imminent threat of terrorist attacks directed at prominent Kenyan facilities and areas where foreigners are known to congregate, such as malls and night clubs."

Measures had been taken to limit official US government travel to Kenya, it said, warning Americans to also consider deferring travel to the country at this time.

A recorded message from the Shebab's leader Mohamed Abdi Godane said Saturday: "The Islamic regions in Somalia are all on high alert to prepare for the open war that is our response to the incursions by some neighbouring countries who are taking part in the global Christian invasion against Somalia."