Somalia's Islamist Shebab insurgents face increasing pressure from regional armies and government forces, with almost every Horn of Africa nation drawn into the two-decade-long conflict.

Here is an overview of key groups in the battle for power:

AL-SHEBAB. The Al-Qaeda-linked insurgents, whose name means "youth" in Arabic, control large areas of southern and central Somalia. They are thought to number around 2,000 full time fighters, approximately one-tenth of whom are foreigners. The Shebab have imposed draconian restrictions on foreign aid agencies, with some of the country's famine zones in areas they control. After pulling out of fixed positions in Mogadishu in August, they switched to guerrilla attacks there, including suicide bombings.

TRANSITIONAL FEDERAL GOVERNMENT (TFG) forces. Thought to number some 3,000 soldiers, but ghost troops boost the payroll to twice that number. Some troops in the national army of the fragile Western-backed government have benefited from training by Western military experts as well as Uganda, but large sections of the rag-tag force are prone to infiltrations and defections.

AFRICAN UNION MISSION IN SOMALIA (AMISOM). The 10,000 strong force is the best-equipped force in the conflict, deployed since 2007 to protect the TFG from the Shebab in the war-shattered capital. The force is split between troops from UGANDA and BURUNDI, while the first 100 men of a promised 850 troop deployment from Somali-speaking DJIBOUTI arrived in December.

ETHIOPIA has an estimated 1,500 men in Ethiopia. The forces were reported to have crossed into central Somalia in November, Addis Ababa's first large-scale incursion since it pulled out in early 2009, more than two years after sending at least 30,000 troops with US blessing. That invasion removed an Islamist administration but sparked a bloody uprising and spurred the emergence of the even more radical Shebab. Ethiopia is believed to have maintained intelligence operatives on the ground and is the foreign force with the best knowledge of Somalia.

KENYA sent tanks and an estimated 2,000 to 3,000 troops in an unprecedented incursion into southern Somalia in October to fight the Shebab. Despite carrying out heavy air strikes progress on the ground has been slow. It has also backed anti-Shebab proxy forces in southern Somalia.

COVERT WESTERN FORCES are also reported to be operating inside Somalia. The United States flies drones out of neighbouring Ethiopia as part of a counter-terrorism campaign, but says the aircraft are for surveillance only.

MULTIPLE MILITIA FORCES, including those backed by regional powers as proxies. These include AHLU SUNNA WAL JAMAA, an anti-Shebab militia some 2,000-5,000 strong, who follow Somalia's traditional Sufi branch of Islam, with ties to Ethiopia. Some factions are more closely allied to the TFG than others. The SHABELLE VALLEY ADMINISTRATION -- a clan-based militia in the Beledweyne area, is also said to be Ethiopian-backed.

KENYAN PROXIES in the south include Ras Kamboni and Azania, who are themselves rival forces and who number about 500 fighters each. RAS KAMBONI is headed by Sheikh Ahmed Madobe, a hardened fighter and a defector from Hezb al-Islam, a group that merged with Shebab in late 2010. AZANIA is headed by Mohamed Abdi Mohamed, also known as Gandi, an academic who spent many years in France.

DOZENS OF FREELANCE GROUPS also operate countrywide, banded together by clan affiliations or opportunistically for tasks such as looting or setting up roadblocks. These groups change allegiance on a regular basis.