DAMASCUS — Rebels trying to topple Syrian President Bashar Assad's regime battled army troops inside an air base in the north Thursday as government forces fought opposition strongholds near the seat of his government in Damascus. The fighting came as Damascus accused joint UN-Arab League peace envoy Lakhdar Brahimi of “flagrant bias” against it, a day ahead of talks he was to hold with top US and Russian officials on Syria's conflict. The sharp broadside against the veteran Algerian diplomat, who has tried since September to quell the violence in Syria, revealed rising diplomatic tensions over the country's 21 months of violence. It came in reaction to Brahimi's comments that a three-stage road map President Bashar Al-Assad proposed last Sunday to negotiate a “political solution” with approved elements of the opposition was “one-sided”. “Syria is shocked by the statements of Lakhdar Brahimi, who has overstepped his mandate and exhibited a flagrant bias for those parties known to be conspiring against Syria and its people,” the Syrian foreign ministry said in a statement broadcast by state television. Brahimi on Wednesday told the BBC that Assad's plan to keep fighting rebel “terrorists” and ignoring opposition groups tied to them, while offering limited dialogue only to domestic opponents deemed acceptable was “not really different and it is perhaps even more sectarian, more one-sided.” In Brussels, a NATO official said the alliance Wednesday detected the launch of an unguided, short-range ballistic missile in the country, which is embroiled in a civil war that has killed more than 60,000 people in nearly 22 months. The official, who spoke to The Associated Press on condition of anonymity in line with the alliance's rules, also said Assad's forces fired ballistic missiles at opposition-held areas twice earlier this month. The official said all the missiles were fired from inside Syria at unconfirmed targets in the north. The alliance has condemned the use of such missiles, saying it disregards the lives of the Syrian people. Syrian state-run SANA news agency said army troops were battling rebel units in several provinces around the country and in the suburbs of Damascus. The agency claimed troops were “inflicting heavy losses on the terrorists and destroying their weapons and ammunition.” The Syrian government refers to rebels as terrorists out to destroy the country and claims they are supported by Arab Gulf countries and the West. In recent weeks, rebels have captured large areas in the north along Syria's border with Turkey and in towns and villages around Damascus. Opposition fighters also have overrun several military bases, seizing weapons and ammunition from government forces, which outgun rebels with their fighter jets and helicopter gunships. The Britain-based opposition activist group Syrian Observatory for Human Rights said the rebels entered the Taftanaz air base in northern Idlib province late Wednesday and the two sides were still fighting on Thursday. Rebels have battled army troops for weeks for control of the Taftanaz base, where helicopters and war planes take off on missions to bomb rebel-held areas around the country. The observatory said the rebel assault was led by fighters from Jabhat Al-Nusra, an extremist group which fights among the Syrian rebels. Also Thursday, nearly 50 Iranians were heading home after being freed by Syrian rebels in a prisoner swap. Iranian officials told reporters at Tehran's Mehrabad Airport that the flight carrying 48 former Iranian prisoners had left Damascus en route to the Iranian capital. They did not say when the plane is expected to land in Tehran. The rebels freed 48 Iranian captives on Wednesday in exchange for more than 2,000 prisoners held by Syrian authorities. It was the first major prisoner swap since the uprising began. — Agencies