(QHA) -

British Prime Minister David Cameron says it is unacceptable for Russian troops to be in Ukraine and warned that Western states will step up sanctions against Moscow if it does not change its behavior.

Cameron spoke to BBC Television before a two-day NATO summit in Wales on September 4-5, at which the alliance is expected to show unity against Russia over the conflict in eastern Ukraine.

Western governments reject Russia's denials that it has sent troops and weapons into Ukraine to help separatists fighting government forces.

Cameron said, "We need to be absolutely clear that what is happening here is unacceptable, Russian troops on Ukrainian soil."

He said sanctions had already had an effect and added, "What Russia needs to understand is if they continue with this approach in Ukraine, this pressure will be ramped up."