A bipartisan group of senators led by Sen. Dick Durbin, D-Ill., is urging international groups to immediately deploy civilian monitors to eastern and southern Ukraine after pro-Russian forces seized control of Ukrainian military facilities in the Crimean region.

The group of senators sent a letter to the Organization for Security and Cooperation in Europe, the wold's largest security-oriented intergovernmental body, asking it to send civilian monitors to eastern, southern and “other potentially unstable regions” of Ukraine to report on security and human rights conditions on the ground and “to help defuse tensions.”

Along with Durbin, Sens. John McCain, R-Ariz., Chris Murphy, D-Conn., Ron Johnson, R-Wis., Sheldon Whitehouse, D-R.I., John Barrasso, R-Wyo., Jeff Flake, R-Ariz., and John Hoeven, R-N.D., all of whom traveled to Ukraine last week, signed the letter.

“We just returned from Ukraine and are deeply concerned about events that are transpiring as well as the broader implications for European security should Russia’s actions go unchecked,” the senators wrote.

They also expressed deep concerns about reports of pro-Russian militias' efforts to intimidate and harass Ukrainians along the eastern border it shares with Russia.

“Russia’s further use of provocateurs and intelligence agents to brazenly stir trouble in eastern Ukraine must be exposed and monitored so as to not provide Russia a manipulated pretext for additional military action in Ukraine," they wrote.

Earlier Wednesday, Reuters reported that the OSCE failed to reach consensus on sending monitors to the region after Russia effectively vetoed the action by the 57-member security watchdog group, which acts on the principle of consensus.

"This is the third time that a text has been presented to which only one state objects — and that is the Russian Federation," U.S. Ambassador Daniel Baer said after OSCE talks broke up Wednesday, according to the report.

Tensions between Ukraine and Russia hit a new high Wednesday afternoon when Ukraine's acting president gave pro-Russian leaders in Crimea three hours to release the detained head of the Ukrainian navy or face “an adequate response.”