* Heaviest fighting in Damascus for months
* NATO agrees to deploy missile defence on Turkish border
* Britain recognises opposition coalition
By Khaled Yacoub Oweis
AMMAN, Nov 20 (Reuters) - Syrian government troops backed by tanks battled to oust rebel forces from an opposition stronghold in a Damascus suburb on Tuesday in the heaviest fighting in the capital for months.
In action in the country's north, rebel fighters stormed an air defence base that President Bashar al-Assad's military had used to bombard areas near the Turkish border.
On the international front, the Turkish foreign minister said NATO states had agreed to supply Turkey with a Patriot missile system to defend against Syrian cross-border shelling.
Although the deployment would be for defensive purposes only, it nonetheless marked a hardening in the foreign opposition to Assad.
The rebels also received a diplomatic lift with Britain officially recognising the opposition Syrian National Coalition, set up this month to boost their chances of securing foreign aid and arms, as the legitimate representative of the Syrian people.
It was the tenth country to do so following France, Italy, Turkey and the Gulf Arab states.
After months of slow progress marked by poor organisation and supply problems, the rebels have captured several army positions in outlying regions in the last week, including a Special Forces base near Aleppo, Syria's commercial hub.
They are also trying to take the 20-month-old revolt to the heart of Damascus, Assad's seat of power, and have dubbed this week "March to Damascus Week".
Elite Republican Guard troops backed by tanks attacked the rebel stronghold of Daraya on the city's southwestern edge and were met by fierce resistance from rebels, who hung on to their positions despite days of aerial bombardment, oppostion sources said.
Twelve people were killed on Monday in Daraya, mostly by aerial bombing preparing for the ground offensive, activists said. Thousands of residents had fled to nearby suburbs.
A Western diplomat following the fighting said Assad had to show he could repel the rebel challenge to Damascus.
"He has to show that letting the bases fall in and round Damascus is only temporary while he begins to consolidate resources and personnel and deals with the struggles in the east," the diplomat said.
Also on Tuesday, two mortar rounds hit the Information Ministry building in Damascus, causing damage but no casualties, state televison said. It blamed "terrorists" for the attack, the usual government term for anti-Assad forces.
On Monday, rebels seized the headquarters of an army battalion near the southern gate of Damascus, the nearest military base to the capital reported to have fallen to opposition fighters.
In the north, opposition sources said rebel fighters had captured sections an air defence base at Sheikh Suleiman, 18 km (11 miles) from the Turkish border and 30 km (20 miles) northwest of Aleppo.
"The fighters have taken three artillery pieces and have entered most of the base. Fighter jets are flying over the area to try and force them out," said Abu Mujahed al-Halabi, an activist with the opposition Sham News Network.
A rebel source said the fighters seized large stocks of explosives and would withdraw to avoid retaliatory air strikes.
"Assad's forces use the base to shell many villages and towns in the countryside. It is now neutralised," the source said.
MISSILES ON THE BORDER
In Ankara, Turkish Foreign Minister Ahmet Davutoglu said NATO states had agreed to supply Turkey with an advanced Patriot missile system to defend against Syrian attacks. Talks on its deployment are in the final stage, he said.
In recent months artillery and mortar fire from Syria has landed inside Turkey, increasing concern that the anti-Assad uprising could turn into a regional conflagration.
Turkey, Gulf Arab states and Western powers have all called for Assad - whose Alawite family have ruled Sunni Muslim-majority Syria in autocratic fashion for four decades - to relinquish power. Assad counts on the support of long-time ally Russia and Shi'ite Iran.
NATO Secretary General Anders Fogh Rasmussen said on Monday that any missile deployment would be a defensive measure and not to enforce a no-fly zone over Syria.
Although the rebels have taken large swathes of land, they are almost defenceless against the government's air force. They have called for an internationally enforced no-fly zone, a measure that helped Libyan rebels overthrow dictator Muammar Gaddafi last year.
Despite strong censure of Assad, Western powers have shied away from direct military involvement.
But the political campaign against Assad took a step forward on Tuesday when British Foreign Secretary William Hague announced that Britain had decided to recognise the new coalition of Syrian revolutionary and opposition forces as the people's sole legitimate representative.