Talks with Russia over NATO's planned missile defence system are failing to progress, the head of NATO said on Thursday, diminishing the likelihood of a summit with Moscow at the NATO summit in May.

NATO is developing the system to counter threats from states such as Iran. But Moscow is seeking legal guarantees that it is not aimed at limiting Russia's strategic nuclear capability, and wants joint control of how the system is used. NATO has declined these demands.

I still hope we will be able to reach an agreement with Russia on missile defence cooperation, NATO Secretary-General Anders Fogh Rasmussen told a news briefing.

However, I also have to make it clear that we have not made much progress so far.

Russia is concerned about plans to station elements of the missile defence system near Russia, in NATO member states Poland, Romania and Turkey.

These elements would include radar that would have coverage extending into Russian territory, something that might give NATO the potential to tamper with Russian defence systems.

Russia has threatened to deploy missiles and radar near NATO territory to counter the system if an agreement to cooperate cannot be reached.

NATO has been pushing for an agreement in time for a summit of NATO leaders to be held in Chicago from May 20-21, which Russia would attend if there was a deal in prospect.

Maybe we will not have a clarified situation until a few weeks before the summit, Rasmussen said.

We still keep it as an option to have a NATO-Russia summit in Chicago. But if there's no deal, probably there will be no (NATO-Russia) summit.

(Reporting By Sebastian Moffett; Editing by David Brunnstrom)