Kenyan fighter jets killed at least 60 Islamist Shebab insurgents in southern Somalia in the latest assault by regional countries to heap pressure on the extremist rebels, officials said Saturday.

"Levels of casualties were very high in air strikes on Friday," Kenyan army spokesman Colonel Cyrus Oguna told reporters, adding the bombing raids hit rebel positions in Garbahare in southern Somalia's Gedo region.

"Provisional casualties are that Al-Shebab lost 60 or more fighters, and more than 50 were injured," Oguna said, adding that nine "technicals" -- pick-up trucks mounted with guns -- were destroyed.

It was not possible to confirm the deaths independently, while the Shebab rebels dismissed the casualty figures as a "fabrication."

Kenya's "short-sighted gunners seem to mistake the trees for Al-Shebab," a rebel message on Twitter read, claiming no insurgent base had been hit.

Shebab fighters control large parts of central and southern Somalia but are facing growing encirclement from government forces and regional armies.

However, the Al-Qaeda-linked group remains a potent threat. Britain's Foreign Office warned its nationals Saturday of a heightened risk of "terrorist attacks" in the Kenyan capital, adding that they "may be in the final stages of planning."

Kenyan police spokesman Eric Kiraithe admitted that despite a boost in security threats remained, urging shopping malls to remain vigilant and to "do more" to prevent attacks.

"We don't believe that Al-Shebab activities are totally neutralised," Kiraithe told reporters, adding that police had thwarted several recent attacks.

Kenya sent troops across the border into Somalia in October to battle the hardline militants it blamed for a spate of attacks on home soil, alongside Somali pro-government forces.

Progress has been slow, with Kenyan forces at first bogged down in mud, but the army has been keen to portray an upbeat message of its chance of success against the insurgents.

"We will keep hitting them until their spine is completely broken ... and we will relish that moment," Oguna said, adding that Kenya's official combat losses so far are six soldiers killed by enemy fire and 22 wounded.

Kenyan forces also seized the village of Fafadon and the village of Elade in Gedo region, he said, while the insurgents face assaults on other fronts too.

A 10,000-strong African Union force, made up of troops from Uganda, Burundi and Djibouti, is defending the fragile Western-backed government from guerrilla attacks by Shebab fighters in the war-torn capital Mogadishu.

Ethiopian forces moved across its border into Somalia in November, and last month fought alongside pro-government gunmen to wrest control of Beledweyne in central Somalia's Hiran region from the insurgents.

Beledweyne is a key trading town leading from the Ethiopian border south into the capital Mogadishu, as well as lying on main route between north and south Somalia.

Kenya said it had exchanged liaison officers with the Ethiopian army since they both face a common enemy, but that the two fronts remained separate.

"The Ethiopians ... might ease pressure because Al-Shebab will be spread out with more enemies to fight, but they have their operation, and we have ours," Oguna said.

Nairobi has proposed its troops "re-hat" to join AU forces, with the pan-African bloc on Thursday saying it will ask the United Nations to authorise an increase of 5,700 troops to bring the force up to 17,700 strong.

With fighting on multiple fronts, the Shebab are believed to be struggling financially, after losing a key source of income when they pulled out of fixed positions in the capital last August.

Now the fighters rely largely for funding on the southern port of Kismayo and the charcoal trade, both of which are under pressure from Kenya.