Thousands of Afghans have crossed into Pakistan
Mr Hassan added that UNHCR efforts to deal with the refugees turned back at the border had been further jeopardized by the seizure of the agency's offices in Spin Boldak by Taleban soldiers.
The UNHCR building housed a field office that provided food and shelter to Afghan refugees coming back from Pakistan.
Closed doors
The Pakistani border has been officially closed to refugees from Afghanistan, but those clearly suffering - often malnourished women and children - have been allowed to enter.
Many of these people had been sheltered at the temporary site at Killi Faizo near the town of Chaman. But on Wednesday morning a sign went up forbidding further access.
"All we have is the sign board saying don't come in, in three languages. We are disappointed and sorry for the people out there," said Mr Hassan, adding that the UNHCR would take the matter up with the Pakistani Government.
The authorities say the camp, currently providing shelter to nearly 2,000 refugees, has exceeded its capacity.
UNHCR chief Ruud Lubbers has repeatedly called on Pakistan to allow more refugees in, promising that his agency would cope with an influx.
But the authorities have so far remained unwilling to relax their policy, worried about the costs and potential for unrest of any flood of refugees.
They note that there are already two million Afghans inside Afghanistan. At least 50,000 have crossed since the air strikes began.
Taleban raid
The seizure of the UNHCR offices at Spin Boldak, one kilometre inside the Afghan border, follows a similar raid on the agency's buildings by Taleban forces in Kandahar.
On that occasion all UN vehicles and equipment were also taken over.
Nobody was in the building when the soldiers arrived on Wednesday morning.
But Mr Hassan acknowledged that the loss of the building would make the work of his agency "very, very difficult".