A request by Turkey for Nato Patriot missile defences to be deployed on its territory followed intelligence that the Syrian government was contemplating the use of missiles, possibly with chemical warheads, Turkish officials have told the Guardian.

The officials said they had credible evidence that if the Syrian government's aerial bombardment against opposition-held areas failed to hold the rebels back, Bashar al-Assad's regime might resort to missiles and chemical weapons in a desperate last effort to survive.

The Turks believe that the regime's Soviet-era Scuds and North Korean SS-21 missiles would be aimed principally at opposition areas but could easily stray across the border, as Syrian army artillery shells and mortars have done.

A missile, especially with a chemical warhead, would represent a far greater threat to Turkish border communities, so Ankara decided last month to ask Nato to supply Patriot missile defence systems, which can spot an incoming missile and intercept it.

"We have intelligence from different sources that the Syrians will use ballistic missiles and chemical warheads," a senior Turkish official said. "First they sent the infantry in against the rebels and they lost a lot of men, and many changed sides. Then they sent in the tanks, and they were taken out by anti-tank missiles. So now it's air power. If that fails it will be missiles, perhaps with chemical warheads. That is why we asked Nato for protection."

The New York Times reported that western intelligence officials had spotted new signs of activity around Syrian military sites where chemical weapons are stored. A senior US official was quoted as saying: "[T]hey're doing some things that suggest they intend to use the weapons. It's not just moving stuff around. These are different kind of activities."

The Syrian regime is believed to have stocks of mustard gas, sarin nerve gas and possibly VX, another nerve agent. Western governments have warned Assad that any use of these weapons would trigger direct military intervention against him. So far, western officials say there are no signs of the regime taking the final steps of preparing chemical artillery shells, missiles or aircraft bombs for use. The deployment of Dutch and German Patriot systems is due to be voted on by those countries' parliaments this week, and Turkish diplomats expect it to be approved. The same two countries supplied the launchers and missiles the last time Patriots were deployed in Turkey, in 2003 during the Iraq war.

In recent days the rebel Free Syrian Army has succeeded in shooting down Syrian government aircraft with shoulder-launched anti-aircraft missiles, in a potential turning point, but Turkey still expects a protracted struggle for the upper hand in the bloody civil war, in which it estimates 50,000 people have died.

Turkish officials still believe the best chance of a breakthrough that would cut short the conflict would be for Russia to withdraw its backing for Assad, forcing the Syrian president, his family and immediate entourage into exile, and thereby removing the most serious obstacle to talks between the opposition and the government.

Russia has blocked any punitive UN security council measures and has supplied the Syrian regime with arms and economic support. In recent weeks it is reported to have flown in tonnes of freshly printed banknotes to allow Damascus to pay its soldiers. But Turkish officials believe Russian backing for the Syrian leader is finally fading. "Privately they have been telling us that they accept he is going to go," a senior official said.

The Russian president, Vladimir Putin, is expected to fly to Turkey on Monday for bilateral talks with the Turkish prime minister, Recep Tayyip Erdogan, in which Erdogan will keep up the pressure for the Russians to pull the plug on their closest Middle East ally. "We are asking the Russians whether or not they want to help build a stable Syria after Assad," a Turkish official said.

A regional peace initiative launched by Egypt's president, Mohamed Morsi, in August, involving Egypt, Turkey, Saudi Arabia and Iran, foundered on Saudi objections to Iranian involvement. Both Egypt and Turkey, however, believe that Iran has to be engaged in the search for a peace deal as it is Assad's only regional ally and an important source of weapons.

Turkey has sustained the effort by organising three sets of trilateral talks: Turkey, Iran and Egypt, whose leaders met in Islamabad late last month to discuss the Syrian crisis; Turkey, Iran and Russia; and Turkey, Egypt and Saudi Arabia. Ankara believes that all those relationships will be vital in rebuilding Syria after the conflict, but that Russia's role will be decisive in bringing it to an end.