The Red Cross says relief workers are being kept out of South Waziristan region, where the Pakistani army is mounting an anti-militant offensive.
A senior official said there was evidence that the level of civilian casualties there was rising sharply.
He added that aid workers faced “very heavy restrictions on access”, mainly because of the heavy fighting.
His comments come on a day that has seen at least 23 people killed in bomb attacks in northern Pakistan.
At least 16 wedding guests – most of them children – were killed when their minibus hit an explosive device in the tribal area of Mohmand, about 35km (22 miles) from the district capital, Ghalnai.
A suicide bomber killed seven other people near an air force base 60km south-west of the capital, Islamabad.
And in Peshawar, a car bombing wounded at least 15 people – the first attack in the city since the army began its offensive in nearby South Waziristan.
The International Committee of the Red Cross’s head of operations for South Asia, Jacques de Maio, said reports from people who had managed to flee South Waziristan – and other areas of northern Pakistan where the army was battling militants – suggested that the number of civilian casualties had surged.
“What we see now is a sharp and extremely worrying increase in the number of civilian casualties,” he told journalists.
“Aid must reach those who need it. We see effective and unobstructed medical services for the sick and wounded as imperative, followed by assistance to IDPs [internally displaced people] and host families.
“To achieve this… humanitarian access must expand and reach a meaningful level.”
Sanctuary
The BBC’s M Ilyas Khan in Islamabad says that aid workers – and journalists – are currently prohibited from entering South Waziristan.
He adds that the army would argue that it is still too dangerous, especially for foreign aid agencies that have been targeted by militants in the past.
Pakistan launched an offensive against the Taliban stronghold in the nearby region of South Waziristan last weekend.
The region is considered to be the main sanctuary for Islamic militants outside Afghanistan. It also has numerous training camps for suicide bombers.
Tens of thousands of civilians have fled the region since the offensive began, according to the army.
There are no details of the latest fighting, as journalists are not allowed into the conflict zone.
Security across Pakistan has been stepped up amid rising militant violence as the offensive continues.
Attacks in cities have killed nearly 200 people in October alone.