Media playback is unsupported on your device Media caption Jeremy Pocock recalls his final moments with his son

The family of a Briton killed in July's MH17 air crash have told of their pain at still not knowing what happened.

Ben Pocock, 20, of Keynsham, near Bristol, was one of 298 people killed when the Malaysia Airlines plane came down in eastern Ukraine, near Russia.

The exact cause is still unclear, but a missile attack is strongly suspected.

Ben's father, Jeremy, said: "The most important question is 'what happened and who did it?'. They must be held to account. That's the bottom line."

Dutch experts say flight MH17 broke up in mid-air after being hit by "objects" that "pierced the plane at high velocity".

Western nations say evidence points to a Russian-supplied missile fired by pro-Russian rebels, but Moscow has blamed Ukrainian government forces.

Image copyright PA Image caption Ben Pocock was studying international business

In the family's first interview since the crash, Jeremy Pocock said he had been informed that about 700 fragments of human tissue or bones from the crash scene were still to be identified - and may not be until next May.

He told BBC Newsnight: "Four Dutch families have not had any identification whatsoever.

"They've had to wait all this time for no news whatsoever and that's absolutely dreadful."

'Desperately sad'

He said even in some cases where identification had happened, it had been "minimal".

"We're talking five months. Not enough to be able to have a funeral and to be able to move forward.

"It's just so desperately sad."

About 500 people attended Loughborough University student Ben's funeral in Keynsham earlier this month.

Mr Pocock said: "We've been fortunate - if that's the right word - in that we've been able to recover quite a lot of Ben.

"But we are one of those families that will almost certainly have to go through the process of dealing with further identifications."

Ben's younger sister, Emily, added: "It's been a very strange process because Ben was supposed to be in Australia for six months, he wasn't supposed to be at home.

"So we've had a hard time actually accepting what's happened. So it was really important to be able to have the chance to say goodbye."

Ben, a keen sportsman, was studying international business.

Image copyright EPA Image caption Crash debris was scattered over a vast area

He had been heading to Australia for six months of study and travel when the plane crashed on the Amsterdam to Kuala Lumpur leg of his journey.

Mr Pocock recalled how he and his wife, Louise, had waved Ben off on his trip at Bristol airport.

"We said our goodbyes and had a hug and that was it," he said.

"We watched him go off into departures and... we never saw him again."

He added how, after news of the crash broke, he had been unable to watch or read reports about it.

"We have a huge pile of newspaper that's been saved for us... even now I can't look at them. It's too painful," Mr Pocock said.

"The fact of seeing his face in the newspapers is just very difficult to deal with."

Image copyright AFP Image caption Journalists pored over the crash site for weeks

Mr Pocock has maintained contact with other British and Dutch families affected by the crash as they press for answers from investigators.

He said he hoped a Dutch reconstruction would "prove conclusively what sort of missile was used".

And he also wants to know why passenger airlines were being allowed to fly over Ukraine given the fighting with pro-Russian rebels in the area.

"It's extremely frustrating and hard to know what theory you can immediately dismiss and which ones you think are more credible," he added.

Image copyright Getty Images Image caption About 500 people attended Ben's funeral earlier this month

"It's very easy to get emotional about this and think the Russian separatists, Putin's got a lot to answer for.

"We have to be careful that we don't jump the gun and that we've got all the evidence and it's properly gathered."

Mr Pocock said it was important to secure justice for all the passengers.

"I take heart from Lockerbie because it took many, many years, but ultimately they did bring somebody to justice over that.

"If they can do it, we can do it and I'm sure we will."

You can watch the full interview with Jeremy and Emily Pocock on Thursday's Newsnight, 22:30 GMT on BBC Two