The United States is stepping up its efforts to allay Russian concerns over its plan to deploy a limited missile defence system in Europe.
The proposal is the focus of talks in Brussels on Thursday that will involve Nato and Russian officials.
Washington says the missile system is intended to protect the US from possible long-range missile attacks from Iran and North Korea.
But Moscow sees the plan as a threat to its own strategic interests.
Nato countries in Europe also have to be fully convinced of the urgency and scope of what the US is proposing, the BBC's diplomatic correspondent Jonathan Marcus says.
'Double sales pitch'
Washington has despatched a high-level team to Nato to set out its case for limited missile defences.
It includes both Under Secretary of Defence Eric Edelman and the Director of the US Missile Defence Agency, Lt Gen Henry Obering.
Their job is to make a double sales pitch, our correspondent says.
On the one hand, he says, the Americans must convince the Europeans that there really is an urgent need to deploy defences against a potential Iranian missile attack.
But equally - at a subsequent meeting of the Nato-Russia Council - they must try to convince Moscow that it is not threatened by the scheme.
So far Russia has responded angrily to Washington's plan to base up to 10 missile interceptors on Polish soil and a new radar system in the Czech Republic.
It sees this as ultimately compromising the effectiveness of Russia's own deterrent. The Americans say it is nonsense given the scale and location of the proposed system.
While there is a broad consensus in Nato that missile defence should be on the alliance's agenda, the Americans are yet to convince everyone of the strategic rationale behind their plan, our correspondent says.
He says that some see potential echoes of the Cold War battles over the deployment of medium-range US nuclear missiles in Europe during the 1980s.
The ruling German coalition is deeply divided on the issue, and the government in Berlin is eager to establish a consensus within Nato before any plans go ahead, our correspondent says.
(BBC News, 19/04/2007)