Story highlights Al-Shabaab says Kenyan planes are killing Somali citizens

"We shall come into Kenya if you do not go back," the group says on a website

Kenya is pursuing militants after recent abductions

Islamic militants in Somalia have threatened to "come into Kenya" if Kenyan forces do not leave Somalia, according to an online message posted on a jihadist website.

"Kenyan troops have entered 100 kilometers into Somalia, and their planes are bombarding and killing residents," said Sheikh Ali Mahmud Ragi, spokesman for Al-Shabaab, an Islamic extremist group considered a terrorist organization by the United States, in the posting. "We shall come into Kenya if you do not go back."

Kenyan forces crossed into Somalia to pursue Al-Shabaab fighters after the recent abductions of tourists and aid workers in Kenya heightened tensions in East Africa. Kenya invoked the United Nations charter allowing military action in self-defense against its largely lawless neighbor.

"If you are attacked by an enemy, you have to pursue that enemy through hot pursuit and to try (to) hit wherever that enemy is," said Kenyan Defense Minister Yusuf Haji in a news conference aired on CNN affiliate NTV on Sunday.

Al-Shabaab, which is linked to al Qaeda, has been fighting to impose its own interpretation of Islamic law, or sharia, on Somalia.

On September 11, armed bandits broke into a beachfront cottage where Britons Judith and David Tebbutt, both in their 50s, were staying. David Tebbutt was shot dead while trying to resist the attack. His wife was grabbed and spirited away onboard a speedboat, and is believed to have been taken into Somalia.

On October 1, pirates made another cross-border raid, this time snatching a French woman in her 60s from a holiday home on Manda Island where she lived part of the year.

Earlier this month, gunmen abducted two Spanish workers from the medical charity Medicins Sans Frontieres (Doctors Without Borders) from the Dadaab refugee complex, about 80 kilometers from the Somali border.

Kenya announced its new tactics days after African Union forces claimed victory against Al-Shabaab in the Somali capital of Mogadishu. The military said last week it had taken the remaining Al-Shabaab strongholds in the far northeast of the city.

"The challenge is now to protect civilians from the sort of terror attack we saw last week, as they attempt to rebuild their lives," African Union Mission in Somalia (AMISOM) spokesman Lt. Col. Paddy Ankunda said. He was referring to a suicide truck bombing in Mogadishu earlier this month that left dozens dead. Al-Shabaab claimed responsibility. Other Al-Shabaab attacks that week led to the deaths of at least 10 civilians.

Federal and African Union forces have battled Al-Shabaab in the impoverished and chaotic nation for years. Many analysts believe the AMISOM military push has severely affected Al-Shabaab, along with targeted strikes against organization members and the weakening of al Qaeda.

Al-Shabaab said in August it was withdrawing from Mogadishu, and Somalia's Transitional Federal Government, backed by African Union peacekeepers, now controls most districts of the capital city, the United Nations office has said.

However, the group still poses a threat, Ankunda has previously said.