Troops from Djibouti have arrived in Somalia's capital, Mogadishu, to bolster the 10,000-strong African Union (AU) force battling Islamist militants.
It is just the third country to contribute to the AU force, which says it needs extra troops to hold territory gained from the al-Shabab Islamists.
Kenya also says its troops in southern Somalia will join the AU force.
Al-Qaeda-linked al-Shabab is fighting to overthrow the UN-backed interim government.
It is estimated to have between 7,000 and 9,000 fighters and controls many southern and central areas of the country.
In August, it announced a tactical withdrawal from Mogadishu after fierce fighting with AU forces.
But the group has continued to detonate bombs in the city, killing five people in an attack at a busy junction earlier this month.
AU commanders say they need up to 20,000 troops to hold on to Mogadishu.
The BBC's Mohamed Dore in Mogadishu says a plane carrying the Djiboutian troops has landed in the city.
The AU mission said 100 soldiers had already arrived, with a further 800 to follow in the next week or so. Until now, the force only consisted of troops from Uganda and Burundi.
"We are desperately in need of military support to eliminate the threat of al-Shabab," said Somali security official Mohamed Abdirahman, according to the AFP news agency.
Other countries that have failed to fulfil promises to send troops include Nigeria and Malawi.
Djibouti borders Somalia and its people speak the same language.
Kenya sent troops in October to pursue al-Shabab after blaming it for a spate of abductions on its side of the border.
The group denies involvement in the abductions.
Somalia has not had a functioning central government for more than 20 years and has been convulsed by fighting between various militias.
The UN says it is the world's worst humanitarian situation, with famine conditions in three southern areas.