Senator Richard Lugar, an Indiana Republican and chairman of the Senate Foreign Relations Committee, and Sen. Barack Obama, an Illinois Democrat, were delayed at an airport in Russia’s Ural Mountain city of Perm on Sunday. They arrived three hours late Sunday in the Ukrainian capital.

The senators and their aides spent three days in Russia visiting sites where warheads are stored before destruction under the US-funded Comprehensive Threat Reduction (CTR) programme, which Lugar authored with former Georgia Senator Democratic Senator Sam Nunn in 1991.

“We are not certain as to why or the particular activity that caused that delay,” Lugar said, according to the Associated Press.. “We are pleased that our flight was able to continue to Kiev, albeit three hours later.”

He said U.S. Embassy officials informed the senators that “an official at the Russian Foreign Ministry has issued an apology this morning.’

No one from the Russian Foreign Ministry could be reached immediately. The US Embassy could not immediately confirm the information, officials there told Bellona Web on Tuesday.

Russia’s Federal Security Service on Monday, however, defended the plane’s delay, saying it was because the Perm airport isn’t part of an Open Skies Agreement, which allows certain planes to bypass inspections, Russia’s RIA Novosti and ITAR-Tass news agencies reported.

The FSB, the agency that succeeded the Soviet-era KGB, said it could only comment on the report within a week’s time. US Embassy officials, however, said the flight was a U.S. military flight, and therefore should have had diplomatic status.

Lugar’s spokesman, Andy Fisher, said that Russian officials had initially refused to allow the plane to take off, and insisted on boarding it. “They did not. The border patrol finally got orders to let us go,” Fisher said, according to AP.

Maksim Zhalayev, deputy head of the border control service at Perm’s Bolshoye Savino airport, accused the senators of refusing to follow border guards’ orders, telling Russia’s Ekho Moskvy radio that was the reason behind the delay.