Countries that "sow chaos" in Syria could suffer from it themselves, President Bashar al-Assad has said in an interview shown on Russian television.
Assad is facing an increasingly militarised uprising, which has claimed thousands of lives across the country since it erupted in March last year.
"For the leaders of these countries, it's becoming clear that this is not 'Spring' but chaos, and as I have said, if you sow chaos in Syria you may be infected by it yourself, and they understand this perfectly well," Assad said in the interview, broadcast on Rossiya-24 TV on Wednesday, referring to the so-called Arab Spring that toppled other long-time rulers.
He again denounced the armed opposition as a gang of "criminals" who he said contained religious extremists including members of al-Qaeda.
Referring to the opposition Free Syrian Army (FSA), Assad said: "It is not an army and it is not free.
"They get money and weapons from abroad from various countries. It is a group of criminals who have for years broken the law and received convictions."
Assad said that many "foreign mercenaries" from Arab countries fighting for the opposition had been killed, but others were still alive.
"They have been captured and we are preparing to show them to the world," he said, without giving further details.
The government says it has implemented a number of political reforms, but opposition groups have rejected the moves as too little, too late, and say that Assad's departure is an unconditional demand.
But he said in the TV interview that the main opposition bloc, the Syrian National Council, did not have "any kind of weight or significance within Syria", and that a majority of the people supported the government.
He said Syrians showed in the May 7 parliamentary elections that they supported what he called the government's policy of reform.
"The polling stations show the opinion of the people. It is a serious message for everyone both inside the country and also beyond its borders," Assad said.
Syrian officials say voter turnout in the election was 51 per cent, However, only limited results have been released.
Meanwhile, hundreds of UN observers have been deployed in the country to monitor compliance with a ceasefire plan negotiated by Kofi Annan, the UN-Arab League envoy.
Assad said that since the observers' arrival, there had been a reduction in "direct confrontation" between the two sides but an increase in "terrorist attacks".
Accusing the West of ignoring opposition violence, he said: "The West only talks about violence, violence on the government side. There is not a word about the terrorists. We are still waiting."
UN observers 'safe'
Assad's televised remarks came as six UN monitors, who were caught up in violence in the northern town of Khan Sheikhoun, were handed back to their UN colleagues on Wednesday by opposition fighters.
"We gave the six with their cars to a UN convoy near the entrance of Khan Sheikhoun. They are all safe, in good heath and on their way to Damascus," Abu Hassan, a Free Syrian Army commander, said by satellite phone from the site of the handover.
In-depth coverage of escalating violence across Syria
Opposition activists said at least 21 civilians were killed in the town, in Idlib province, on Tuesday after security forces opened fire on a funeral procession while the observers were there.
A spokesman for the UN mission in Syria said three UN vehicles were damaged by an improvised explosive device, but no UN personnel were injured.
A pro-government TV station said unidentified attackers opened fire at the funeral.
The head of the monitoring mission, Major-General Robert Mood, confirmed the monitors were heading back to base.
"They have departed from Khan Sheikhoun and are on their way back. They expressed to me that they have been well treated," he said in Damascus on Wednesday.
He expressed gratitude to the Syrian government for "facilitating co-ordination" for the exit of the observers, and to the people of Khan Sheikhoun for treating them "with respect".
"That kind of violence is obviously the kind of violence we don't want to see," he said. "It is not going to contribute constructively to the aspirations of the Syrian people."