By Gleb Garanich

SUKHA BALKA, Ukraine, Aug 24 (Reuters) Grieving relatives, their heads bowed, gathered today by a meadow where a Russian airliner bringing holidaymakers back from the seaside crashed in eastern Ukraine, killing all 170 on board.

As flags flew at half staff to mark a day of mourning throughout Russia, a wooden cross with black ribbons stood by the gully where the Soviet-designed Tu-154 slammed into the ground on Tuesday after flying into a violent storm.

Mourners were confined to an adjacent hill as search teams from Ukraine and Russia completed clean-up work, gathering debris as well as the remains of those who had been on board.

Teams of medics stood at the ready by a large tent to treat distraught relatives.

The process of identifying the dead was also under way.

Officials said blood tests were required in some cases as the plane was engulfed by a fire once it hit the ground, disfiguring passengers' bodies.

Teams patrolled the site with sniffer dogs to ensure all human remains had been removed.

''We are conducting checks,'' said Vadim Seryogin, head of a team of Russian investigators at the site. ''But we believe that, along with our Ukrainian colleagues, we have got them all out.'' Clergymen were allowed to place flowers and soft toys at the heart of the site, next to a burned out engine. Forty-five of the dead were children, six of them under two years old.

The clergy then scooped up a large sack of earth as a keepsake for mourners, flown in from Russia's second city of St Petersburg, where the aircraft had been headed from Anapa on the Black Sea.

HELPLESSNESS ''The most dreadful thing when you have loss of life is the feeling of helplessness,'' President Vladimir Putin told Spanish King Juan Carlos at his Black Sea holiday residence.

''There is nothing you can do. In a case like this, we remember those who died and try to help those close to them.'' In St Petersburg, where many of the victims lived, mourners led by the regional governor lit candles at a memorial service in the colonnaded Kazan Cathedral.

The Tu-154's ''black box'' data and voice recorders were in Moscow undergoing examination to determine what caused the plane to plunge more than 11,000 metres. Officials said it could take more than a month to decipher the data.

Officials initially said the plane, operated by Pulkovo Airlines, one of Russia's largest, had been hit by lightning.

But they have since refused to offer any explanation pending a full investigation.

Russian media, quoting experts, said the plane may have encountered difficulty by exceeding its maximum allowed altitude to avoid the storm on its route across Ukraine's eastern tip.

Passenger lists showed two Germans, a Dutch national, a French citizen and one Finn among the dead.

REUTERS LL HT1805