Russia said Friday it may boycott a May summit with NATO if the two sides failed to bridge their differences over a US-backed missile defence system for Europe.

Deputy Foreign Minister Sergei Ryabkov said a final decision on attending the Chicago summit will be reached after March 4 presidential polls at which Prime Minister Vladimir Putin is expected to secure his return to the Kremlin.

"It would be easier to stage the summit if we agreed on missile defence by that point," Ryabkov told Russian state television. "We do not know if we will agree or not."

"The staging of such a high-level summit depends first and foremost on whether it is fillled with meaningful content."

Ryabkov's comments came one day after both sides stuck to their guns at a Brussels NATO meeting attended by US Secretary of State Hillary Clinton and Russian Foreign Minister Sergei Lavrov.

Clinton said the United States "will continue to press forward on missile defence" despite a warning from President Dmitry Medvedev about Russia deploying new missiles if the system is fully deployed.

"It does not affect our strategic balance with Russia and it's certainly not a cause for military counter-measures," Clinton said.

Washington argues the shield is designed to protect against potential attacks from Iran amid worries about the Islamic state's nuclear programme.

But Russia fears the system could one day make its own shrinking nuclear arsenal ineffective and has outlined a series of retaliatory steps it may take should NATO ignore its concerns.

The Russia-NATO Council was created in 2002 to help address Moscow concerns about the miltiary bloc's eastward expansion following the Soviet Union's collapse.