ABOARD AIR FORCE ONE: The United States said Tuesday that a “desperate” President Bashar al-Assad was slowly losing his grip on power, citing defections and fighting raging increasingly close to Damascus.

Washington offered new support to its Nato ally Turkey after Syrian forces shot down one of its fighter planes and also pushed back on Russia's insistence that Iran should take part in a planned international conference on Syria.

“Clearly, Bashar al-Assad has been slowly—too slowly—losing his grip over his country,” White House spokesman Jay Carney said on Air Force One as President Barack Obama flew to a campaign event in Atlanta.

“I would note that recent high-level military defections to Jordan and Turkey are another testament to the regime's loss of control over the situation in Syria. It is clear, however, that Assad is desperate to hang on to power at all cost, as evidenced by his continued use of air power and Shabiha gangs.”

A steady stream of high-ranking military officers have defected to Turkey in recent days and several pilots have made their way to Jordan.

Carney said it was also a “fair assessment” that deadly combat around elite Republican Guard posts in the Damascus suburbs between rebels and army units was a further sign of Assad's declining authority over his country.

And he praised Turkey for its “measured response” to the shooting down of its Phantom F-4 fighter jet, which prompted Nato to condemn Syria.

“The United States and Nato stand in solidarity with Turkey. We will work with Turkey and other partners to hold the Assad regime accountable.”

In Washington, State Department spokeswoman Victoria Nuland told reporters that the United States was ready to consider further support to Turkey.

“Turkey is our ally. As Nato said today and as the secretary general (Anders Fogh Rasmussen) said today, we're prepared to look at any requests that Turkey wants to make,” Nuland said.

UN peacekeeping chief Herve Ladsous meanwhile told the Security Council on Tuesday that the world body's operation in Syria would remain suspended because fighting between opposition and government forces was intensifying.

UN-Arab League envoy Kofi Annan has still not secured agreement on a political transition plan that all the major powers can back so that an international meeting on the conflict can go ahead this week, diplomats said.

Violence now in its 16 month has killed more than 15,000 people, the Syrian Observatory for Human Rights says.

“Thousands and thousands of Syrians have paid for Assad's hubris with their lives,” Carney said.

Russia is pushing for an international Syria conference and has already discussed the plan with Jordan as well as the European Union, Iran and Iraq.

President Vladimir Putin's strident rhetoric and a flat-out refusal to support sanctions against Moscow's Soviet-era ally Syria have pitted him against the West.

“We've had very productive meetings” with Russia, Carney said, but added: “There's no question we have differences.”

The US has opposed Russia's call for Iran to be involved in international meetings on Syria.

Secretary of State Hillary Clinton spoke to Annan on Tuesday to work on what a conference might look like and to see how it could facilitate a transition in Syria that Washington says must take place without Assad.

“I would say today that we are getting closer, but we don't have any decisions yet,” Nuland said.

But she signalled Washington was not ready to yield on the issue of Iran's attendance.

“Our view on Iran's participation has not changed given its support for the regime and its continued behaviour vis-a-vis Syria. We just don't see it as able to make a helpful contribution right now,” she said.

Syria is at the top of Clinton's agenda as she left Tuesday for a trip that will take her to Russia on Thursday, and also includes stops in Finland and Latvia.