By Mehreen Zahra-Malik and Jibran Ahmed

TORKHAM BORDER CROSSING, Pakistan (Reuters) - Hundreds of trucks lined roads leading to the normally bustling Torkham crossing between Pakistan and Afghanistan on Thursday, after deadly clashes between the countries forced it to close.

With the smell of rotting fruit filling the air, drivers expressed frustration as their cargoes perished in the heat due to an impasse between the countries over their disputed frontier.

Four people were killed in the fighting on Sunday, an escalation of the dispute over Pakistan's plan to build a barrier at the crossing to stop Islamist militants coming in from Afghanistan.

That plan has angered Afghanistan which rejects the colonial-era Durand Line border drawn up in 1893 and does not want a solid recognition of the boundary.

"We're just sitting here waiting for some good news," said Musa Jan, an Afghan truck driver from the northern city of Mazar-i-Sharif, stuck since Sunday night on the Pakistani side of the crossing, on the edge of the Khyber Pass through the towering Spin Ghar mountains.

"We just want this to end so we can go home and get back to making an honest living."

Hundreds of brightly painted trucks lined both sides of the road, providing shelter from the sun for drivers, many of whom had crawled under their vehicles to wait.

Thousands of vehicles normally pass through the crossing every week, making it a vital trade link between the countries.

The weekend skirmishes have strained an already tense relationship between Afghanistan and Pakistan.

Afghanistan, struggling to contain a stubborn insurgency led by Taliban militants, blames Pakistan for harboring fighters and allied networks on its territory.

Pakistan denies it supports militants, and says it is building the gate at Torkham to stop the movement of militants coming the other way, from Afghanistan.

"In the past there would be small skirmishes, but nothing like this," said one guard at the crossing, Shafi Ullah, referring to the weekend clashes.

"But we will build this gate at all costs ... There is terrorism, there are blasts – it all comes from that side (Afghanistan)."

Pakistan's top foreign policy official, Sartaj Aziz, told parliament on Thursday that Afghan forces had started the clashes.

"There has been no aggression or provocation by Pakistan," Aziz said.

Afghan officials, however, blame Pakistan.

"The past three times, Pakistani forces attacked us, and our forces were in a defensive position and defended themselves and our soil," said Attaullah Khogyani, spokesman for Afghanistan's Nangarhar province where Torkham is located.

Afghanistan's ambassador in Islamabad, Omar Zakhilwal, accused Pakistan of reneging on ceasefire terms that he said included halting construction of the gate.

Zarwaid Khan, a Pakistani border guard, said that construction was continuing and there was no fighting on Thursday, although he said the situation was still "delicate".

"We are watching and waiting," he said.

(Additional reporting by Ahmad Sultan in NANGARHAR, Afghanistan and Asad Hashim in ISLAMABAD; Writing by Kay Johnson; Editing by Mike Collett-White and Robin Pomeroy)