By Richard Balmforth and Pavel Polityuk KIEV (Reuters) - Ukraine accused Russia on Thursday of bringing troops into the southeast of the country in support of pro-Moscow separatist rebels. Ukraine's security and defense council said the border town of Novoazovsk and other parts of Ukraine's south-east had fallen under the control of Russian forces who together with rebels were staging a counter-offensive. "A counter-offensive by Russian troops and separatist units is continuing in south-east Ukraine," the council said in a post on Twitter. President Petro Poroshenko, in a statement explaining his decision to cancel a visit to Turkey, said: "Russian troops have actually been brought into Ukraine." Russia denies intervening in Ukraine by arming the rebels or sending soldiers across the border.