A handicapped Frenchwoman who was kidnapped from her beachfront home at a Kenyan resort island and taken to neighboring Somalia has died, France said on Wednesday.

"The contacts through which the French government was seeking to obtain the release of Marie Dedieu, held in Somalia since October 1, have announced her death, but we have not been able to determine the date nor the circumstances," the foreign ministry said in a statement.

A car bomb exploded in Mogadishu Tuesday near a meeting involving two Kenyan ministers after Shebab rebels promised Nairobi all-out retaliation for its unprecedented military operation inside Somalia.

Two days after declaring war on Somalia's Islamist insurgency, which it blames for a string of kidnappings, Kenya's forces advanced on a strategic rebel-held town deep inside southern Somalia.

Kenyan troops using aerial cover advanced Tuesday on a strategic rebel-held Somali town, even as Nairobi police stepped up security for fear of reprisal attacks by Islamists.

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

Two Britons have been arrested in Kenya near the border with Somalia, police in Wales said Tuesday.

The pair, believed to be from the Welsh capital Cardiff, were detained on Sunday, according to the Foreign Office in London, which said it was ready to provide consular assistance if required.

Kenyan troops have crossed the border into war-torn Somalia to attack Islamist Shebab rebels they accuse of being behind several recent kidnappings of foreigners, Kenyan officials said Sunday.

"We have crossed into Somalia in pursuit of the Shehab, who are responsible for the kidnappings and attacks on our country," government spokesman Alfred Matua told AFP.

Somalia's Shebab rebels vowed more attacks after a deadly car bomb killed more than 70 people in Mogadishu, spokesman Ali Mohamud Rage said in speech broadcast Wednesday by the group's radio.

"We are promising that attacks against the enemy will be routine, more in number and will increase day by day," Rage said over radio al-Andalus.

Gunmen who snatched a disabled 66-year-old Frenchwoman from her beachfront home in a prized Kenyan resort have taken her to neighboring war-torn Somalia, officials said Sunday.

With Kenyan forces obliged to stop their pursuit at the border and officials suspecting Somalia's al-Qaida-inspired Shebab, chances dimmed of a quick release for the woman, named as Marie Dedieu.

Heavy fighting broke out Friday close to Somalia's border with Kenya as extremist Shehab fighters launched a dawn attack on the town of Dhobley but were later repelled, fighters on both sides said.

"The fighting was very heavy but we emerged victorious, there are casualties on both sides," a senior commander of the Azania militia group controlling the area told Agence France Presse by phone from the battleground.

The humanitarian situation in Somalia remains "highly critical," as the war-torn nation struggles from drought and famine, the International Committee of the Red Cross (ICRC) warned Wednesday.

"The situation in Somalia remains highly critical. Hundreds of thousands of Somalis are still displaced from their homes in search of security and food," the ICRC said in a statement.

At least one person was killed Saturday by a powerful explosion at the offices of the United Nations Mine Action Service in Somalia's capital Mogadishu, witnesses said.

The blast destroyed a car in the service's parking lot, killing at least one person in the vehicle, witness Abdulahi Ahmed told Agence France Presse.