His remarks came as the Syrian Observatory for Human Rights said 18 civilians were killed and 40 wounded in “probable” Russian air strikes on a northwestern town held by a rebel alliance.
The Britain-based Observatory also reported that the Islamic State group had murdered more than 3,500 people in Syria, including 2,000 civilians, since declaring its “caliphate” last June.
The latest developments came as Britain pressed efforts to widen its participation in a US-led air coalition battling IS in Iraq to include Syria.
Defence Secretary Michael Fallon said his government was intensively lobbying the opposition Labour party to support air strikes in Syria, two years after it had blocked that option.
In remarks during talks with a senior Iranian official in Damascus, Assad spoke of “important gains by the Syrian army in the fight against terrorism, with the support of its friends led by Iran and Russia.”
State news agency SANA also quoted him as saying that these gains had “pushed certain countries hostile to Syria and who pretend to fight terrorism to… increase their financing and arming of terrorist groups”.
Assad did not identify any country by name but since the outbreak of the Syrian conflict in March 2011, he has accused Turkey and Gulf nations of arming his mainly Sunni opponents.
Assad’s regime, dominated by the Shia Alawite sect, launched a key offensive to retake areas seized by opponents after Moscow began in late September an air war in support of the government.
Iran, the region’s main Shia power, has also backed Assad, including with the reported deployment of hundreds of “military advisers” to bolster regime forces.