(Adds U.S. State Department expressing concern; details, paragraphs 6-11 and 13-16)
By Anton Zverev
DONETSK, Ukraine, July 18 (Reuters) - Gunmen prevented monitors from the Organisation for Security and Cooperation in Europe from observing the site where a Malaysian airliner crashed in rebel-held eastern Ukraine on Friday, the rights and security watchdog said.
Calling their behaviour "impolite and unprofessional", an OSCE spokesman said some gunmen in the area seemed intoxicated while others would not let the team of about 25 observers look at the wreckage of the Boeing 777.
"We had expected unfettered access, that's the way we work," Michael Bociurkiw told a news conference.
"Unfortunately the task was made very difficult. Upon arrival at the site ... we encountered armed personnel who acted in a very impolite and unprofessional manner. Some of them even looked slightly intoxicated."
He denied that the observers had been fired at by pro-Russian rebels, but said one gunman fired shots into the air, seemingly to scare off some civilians.
Earlier, the OSCE's permanent council chairman, Thomas Greminger, told Reuters monitors had not been able to secure an access corridor for the crash-site and that investigators had stayed there for only about 75 minutes before setting off back to Donetsk.
The United States said it was "very concerned" about the way the OSCE monitors had been treated.
"Those who say they are going to participate in or welcome this investigation need to give unfettered access and obviously, we didn't see that when these individuals were there for 75 minutes," U.S. State Department spokeswoman Jen Psaki told a regular news briefing in Washington.
The United States wanted to see "a credible, international investigation," she said, adding that the U.S. government had offered to send personnel and resources from the National Transportation and Safety Board and the Federal Bureau of Investigation.
Psaki said the Ukrainians had accepted the offer and the NTSB and FBI would each send at least one investigator, although it was unclear when they would go.
"And we of course will be responsive to their needs moving forward," she said.
World leaders have called for a rapid investigation into the shooting down of the airliner, which could mark a pivotal moment in deteriorating relations between Russia and the West. The United States and Britain said a surface-to-air missile appeared to have been fired from rebel held territory.
There were no survivors from the Malaysia Airlines flight MH17 from Amsterdam to Kuala Lumpur, a Boeing 777. The United Nations said 80 of the 298 aboard were children. It was the deadliest attack on a commercial airliner and scattered bodies were seen over miles of rebel-held territory near the border with Russia.
U.S. Ambassador to the United Nations Samantha Power told a meeting of the U.N. Security Council on Friday it was unlikely the separatists could have effectively operated that missile system without help from knowledgeable personnel.
Russia's U.N. Ambassador Vitaly Churkin called for an "impartial, open investigation of what happened," and warned against any attempts to prejudge the matter.
Bociurkiw said the OSCE intended to visit the site again on Saturday and spend the whole day there. "We unfortunately could not get much done today because of the behaviour of the armed individuals and the lack of access," he said.
He said the team could not find anyone to talk to about the airliner's black box, and that it was not clear who was in charge of the territory where the airliner crashed.
"The crash site is a very large area and there may be more than one group who holds sway over that area," he said.
He added that while the bodies had not been touched - they seemed to be lying where they fell, personal possessions appeared to have been arranged in piles as if "for show".
Some of the bodies showed early signs of decomposition.
"As for observing close up the wreckage of the Boeing 777 that was very difficult. The armed guards did not allow us very much leeway to leave the roadway and look at the wreckage," he said. (Additional reporting by Georgina Prodhan in Geneva and David Brunnstrom in Washington; writing by Elizabeth Piper, editing by Philippa Fletcher and Bernard Orr)