* Clashes reflect Syria conflict, wider regional divisions
* Fighting between neighbourhoods started in 1980s
* Unlikely to end despite army moves to end bloodshed
* Soldiers arrest 21 militiamen in Tripoli
Clashes resumed on Tuesday between Lebanese militias who back opposing sides of Syria's war and 21 fighters were arrested by the army as it pursued a six-month-long mandate to end bloodshed battering the city of Tripoli.
The conflict between the majority Sunni Muslim Bab al-Tabbaneh district and the adjacent Alawite neighbourhood of Jebel Mohsen in Tripoli has killed over 100 people this year. But residents, fighters and a local politician told Reuters on Tueaday it was unlikely to end soon despite army efforts.
The two neighbourhoods have been in on-off conflict since the 1980s but the 2-1/2-year-old civil war in neighbouring Syria pitting Alawite President Bashar al-Assad against majority Sunni rebels has opened old wounds on both sides in Tripoli, and fighting has become more frequent and intense.
"They (Alawites) are using big mortar bombs now," a teenage fighter from Bab al-Tabbaneh said on Tuesday, showing pictures on his mobile of himself holding assault rifles with Sunni Islamist slogans written behind him.
The 19-year-old refused to give his name while sheltering from the rain in the Taqwa mosque, one of two Sunni religious compounds hit by bombs in August that killed 42 people and angered Sunni fighters even more.
Over the weekend, the relatives of the car bomb victims protested in a Tripoli square, demanding that leading Alawite political leaders be arrested and calling for Jebel Mohsen's electricity and water supplies to be cut off.
The latest clashes started after repeated attacks on Alawite targets over the last week in which several people were wounded. Ten people were killed over the weekend. The army provided no details on the 21 militiamen seized by soldiers.
Reuters was unable to speak to fighters and residents in Jebel Mohsen because the roads to it were cut off by sniper fire on both sides.
Analysts say that the seemingly pointless battle in which neither side gains ground is being directed by regional powers who fund militia to send political messages and assert their control over Lebanon, a weak sectarian-run state wrecked by its own civil war from 1975 to 1990.
"Lebanon is not a sovereign country," said Beirut-based political scientist Hilal Khashan. "Each sect has foreign patrons and they know they need foreign patrons; this country is run from the outside."
Misbah Ahdab, a Sunni Tripoli politician from a secular party, said the local battle was a "regional fight between Iran and the Gulf".
Sunni Saudi Arabia is locked in a struggle with Shi'ite Iran for influence across the Middle East. In Syria, Saudi Arabia and other Gulf countries support the rebels while Iran backs Assad, whose Alawite sect is an offshoot of Shi'ism.
Tiny, coastal Lebanon has suffered from violent spillovers of the Syrian conflict, especially in Tripoli.
Barring a wider regional political compromise, Ahdab said, no security measures could stop the fighting in Tripoli, especially as militia on both sides had been given "protection" by Lebanese politicians aligned with either the Gulf or Iran.
Sheikh Bilal, a middle-aged Sunni fighter, questioned the government's credibility in imposing security in the city. "We have had 700,000 'security plans' before (but) these are all lies."
The fighting has worsened the plight of local residents. A former industrial centre and 70 km (40 miles) from the capital Beirut, Tripoli is now plagued by poverty and unemployment.
A toy shop owner who works near the front line opened his shutters on Tuesday to speak to journalists but said that business had all but ended. "Everyone knows the criminals who are fighting but they are not arrested. They are protected."