BEIRUT — Making diplomatic and military advances, a Syrian opposition coalition gained official recognition from Britain on Tuesday and showed off one of its largest hauls of heavy weapons from a captured government base inside Syria.
The developments came against a backdrop of steadily increasing violence in the capital, Damascus, with expectations growing of a full conflagration there.
In London, Foreign Secretary William Hague told Parliament that Britain had decided to recognize the recently formed coalition as the ‘‘sole legitimate representative of the Syrian people.’’
The coalition, whose official name is the National Coalition of Syrian Revolutionary and Opposition Forces, has already been recognized by France, Turkey, and the members of the Gulf Cooperation Council.
There has been some hesitation in recognizing the new coalition before it proves it can unite the exiled opposition groups with those fighting inside Syria, and organize much-needed humanitarian relief.
But the countries that have extended early diplomatic recognition are making a calculation that backing the coalition now will help make it credible and bolster its chances of becoming a legitimate alternative to the government of President Bashar Assad. The previous attempt, the Syrian National Council, got lost in a thicket of personal bickering.
Supporters of the opposition feared that the political forces in exile were growing increasingly irrelevant while jihadist fighters gradually took on a more prominent role in the uprising against the government.
“It is strongly in the interests of Syria, of the wider region and of the United Kingdom that we support them and deny space to extremist groups,’’ Hague said.
He added that a ‘‘credible alternative’’ to the Assad government was emerging, but that if a political and diplomatic solution was not found, Britain ‘‘will not rule out any option in accordance with international law that might save innocent lives in Syria.’’
Questions swirled around a video posted online on Sunday appearing to show several Islamist groups disavowing the coalition. But at least two of the groups named in the video later said it did not reflect their opinion, reinforcing the murky details surrounding the groups fighting the Syrian government.
In Ankara, the Turkish foreign minister, Ahmet Davutoglu, said NATO states had signed off on deploying an advanced Patriot missile system to Turkey to defend against Syrian attacks.
The missiles, which could come from the United States, were meant for defensive purposes. But the agreement still represented another notch in hardening Western positions toward military action against Syria. Fighting along the Syrian border has repeatedly spilled over, with artillery and mortar fire landing inside Turkish territory.
But in an apparent opposition victory, rebels captured a large military base near Aleppo over the weekend, helping solidify their control over a growing strip of land along the border that many opposition supporters hope will become fully liberated.
Video posted online showed fighters identifying themselves as a brigade belonging to the Free Syrian Army overrunning the base, used by the military’s 46th Regiment, in the Atareb area.
In Damascus, fierce fighting rocked many neighborhoods.
In a symbolic strike at a high-profile symbol of the government, two mortars struck the Information Ministry, which also houses the publication department of the ruling Baath Party.
One fell in the parking lot while the other hit the facade of a building along a main thoroughfare in the middle-class Mezze neighborhood. There were no injuries, according to a Syrian state television report.