Russia apologized Monday to two U.S. senators who were detained for three hours in an airport while officials refused to let their U.S. military flight take off.

Sens. Richard Lugar (search), R-Ind., and chairman of the Senate Foreign Relations Committee, and Barack Obama (search), D-Ill., arrived in the Ukrainian capital three hours later than expected after being delayed by border guards at an airport in Russia's Ural Mountains city of Perm on Sunday night.

After several hours stuck in the airport, and once the military flight's diplomatic status was verified, the senators were allowed to leave.

"We regret the misunderstanding that arose and caused an inconvenience to the senators," Russia's Foreign Ministry said in a statement.

The Foreign Ministry said that the delay, which it said "was incorrectly called a detention," arose because of questions over whether the international flight en route to Kiev had undergone the necessary procedures.

Earlier Monday, Lugar told journalists in the Ukrainian parliament that he had received no explanation for the Russian government's actions but was pleased his flight was allowed eventually to take off.

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

Russia's Federal Security Service (search) defended the 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, which is the main successor agency to the Soviet-era KGB, said it could only comment on the report within a week's time.

U.S. Embassy officials 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 insisted they be allowed to board the flight. "They did not. The border patrol finally got orders to let us go," Fisher said.

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

The senators and their aides spent three days in Russia visiting sites where warheads are stored before destruction under the U.S.-funded Comprehensive Threat Reduction (search) program.

"If this is our strange way of ending or discrediting a program that is crucial to us, it is really silly," said Alexander Golts, a defense analyst with the online magazine Yezhednevny Zhurnal.

Moscow should be highly interested in Lugar's program, which provides hundreds of millions of dollars a year to help Russia destroy weapons of mass destruction, Golts told Ekho Moskvy radio.

"It is also silly if this is our strange and rude way of sending Lugar a note that his statements regarding Yukos and problems with democracy in Russia ... don't suit the Kremlin," Golts said.

Earlier this year, Lugar criticized Russia for its alleged retreat from democracy, warning that a positive relationship with Moscow was complicated because "basic freedoms are being violated in Russia." The Kremlin has also faced criticism for the dismemberment of the Yukos oil company and the jailing of its founder, Mikhail Khodorkovsky.

Mikhail Margelov, head of the Russian upper house of parliament's foreign relations committee, questioned whether the detention of the senators' flight "was a right and useful thing to do" but also said he didn't expect it to lead to a major fallout between the two nations.

On Monday, Lugar and Obama met with Ukrainian Parliament Speaker Volodymyr Lytvyn and President Viktor Yushchenko, a Western-leaning reformer who came to power after massive street protests last year.