Kenyan troops advanced towards a strategic rebel-held Somali town on Tuesday as heavy air strikes battered al-Shabab militant positions, army officials said.

"Our forces will be concentrating on operations in Afmadow region today, they started moving there late on Monday," army spokesman Major Emmannuel Chirchir said.

Kenyan troops have pushed at least 120km into Somalia since Nairobi declared war on the al-Shabab militia and confirmed it had sent its army across the border on Sunday.

Guided by pro-government Somali forces, backed by heavy aerial bombardments but bogged down on mud tracks in heavy rain, Kenyan troops have been hitting back against recent kidnappings inside Kenya blamed on the Islamists.

"So far everything is going on well," Chirchir said.