* U.S. calls on Russia to secure monitors' immediate release
* Russian envoy says was "irresponsible" to go to tense area
* But their liberation would be "good step" to ease conflict (Adds U.S. comment, paragraph 8, other incidents, 15-16)
By Fredrik Dahl
VIENNA, April 28 (Reuters) - A Russian ambassador said on Monday it had been "extremely irresponsible" to send international military monitors to eastern Ukraine, where they were detained by pro-Russian rebels last week.
Andrey Kelin, Moscow's ambassador to the Organisation for Security and Cooperation in Europe (OSCE), said however it would be a good step towards easing the conflict to release the seven European monitors, who are being held in the city of Slaviansk.
Asked what Russia was doing to help achieve that, Kelin told reporters at the Vienna-based OSCE: "We are doing some steps, not only by statements, but also in practice."
He gave no further details.
A senior U.S. diplomat called on Russia to secure the "unconditional and immediate release" of the German-led team.
"We remain disappointed that senior officials in Moscow have not condemned the abduction - nor have they demanded the team's immediate release," Gary Robbins, deputy head of the U.S. mission to the OSCE, told an extraordinary meeting of the 57-nation European security body.
The captives - from Germany, the Czech Republic, Denmark, Poland and Sweden - were paraded before reporters on Sunday and said they were in good health. A Swede was freed for medical reasons but the others are still being held, described by the rebel leader as "prisoners of war" and NATO spies.
In Washington, State Department spokeswoman Jen Psaki said "we are outraged by the shameful display of the international monitors ... as they were paraded in front of the press."
CIVILIAN MONITORS BRIEFLY HELD
The United States says armed rebels - who have captured towns and government buildings across eastern Ukraine - are operating under the direction of Kremlin agents. Russia denies it is involved and says the uprising is a spontaneous response to oppression of Russian speakers by Kiev.
Kelin said people in eastern Ukraine were under the "threat of military operation", making clear he meant by the Kiev government which he suggested had some 11,000 troops as well as hundreds of tanks and military vehicles at its disposal.
Residents were "expecting an aggression ... every moment, so it is extremely tense. That is why it was extremely irresponsible to direct (monitors) to this region in this tense situation," Kelin said after the closed-door OSCE meeting.
It was "quite an adventure, I would say, or a provocation ... to bring these people to a hot spot," he added.
Germany urged Moscow earlier on Monday to use its influence on the separatists to secure the release of the observers.
Kelin said an OSCE mission was doing a lot "to get them free" and that Moscow felt that their liberation would "be a good step for de-escalation of the conflict".
In another sign of rising tension, the OSCE said members of a separate mission of civilian monitors were involved in two incidents on Sunday. That mission, now numbering more than 150 monitors, was approved by all OSCE member states including Russia in March to help defuse the crisis.
A group of unarmed men saying they were from a local militia detained two monitors in the Donetsk region and held them for more than three hours on Sunday, the OSCE said on Monday.
At around the same time elsewhere in the Donetsk area, five heavily armed people cross-examined another monitoring team, accusing them of espionage and briefly holding them, it said. (Additional reporting by Doina Chiacu in Wsashington; Editing by Tom Heneghan)