Russian Foreign Minister Sergei Lavrov has stated that initial US plans to deploy a regional missile defense shield in Bulgaria don’t threaten Russian nuclear forces.

Lavrov however added Tuesday that Russia may back out of the START 2 nuclear weapons pact, if the US continues to step up its missile defense shield program beyond Bulgaria and Romania. “Linkage to missile defense is clearly spelled out in the accord and is legally binding,” he continued.

With respect to Bulgaria and Romania Lavrov said; “we think that when consultations are under way as ordered by the presidents of Russia and the U.S. between Russian and American experts on issues of missile defense, it would be best to avoid such surprises.”

Lavrov’s comments come ahead of the planned signing by Obama and his Russian counterpart, Dmitry Medvedev, of the new nuclear arms reduction pact in Prague on Thursday

The pact, agreed last month, commits Russia and the US to big cuts in nuclear warheads and would replace the 1991 Strategic Arms Reduction Treaty (Start), which expired last December.

The new treaty restricts both Russia and the US to 1 550 warheads, about 30% less than currently allowed, the US says.