More than 70 humanitarian groups have suspended information-sharing cooperation with the UN in Syria, accusing the world body of being influenced in its operations, including in the distribution of aid, by the government of President Bashar al-Assad.

In a scathing letter sent to the UN on Thursday, 73 groups announced their withdrawal from the key "Whole of Syria (WoS) information-sharing mechanism".

Besides suspending participation in the programme, in which groups share information to help in the delivery of aid across Syria, the letter's signatories also called for a transparent investigation into the "political impact that the Syrian government has on humanitarian actors".

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The UN is accused of complacency amid Syrian government interference in the delivery of humanitarian assistance, including blocking aid to besieged rebel-held areas, removing medical aid from convoys, and marginalising humanitarian workers for political reasons, according to a copy of the letter sent to Al Jazeera.

UN officials in Syria are aware of the situation and remain silent, the letter states.

"This deliberate manipulation by the Syrian government and the complacency of the UN have played hand-in-hand. The people of Syria have suffered ever more as a result."

Aid groups such as the Syrian American Medical Society (SAMS) and the Syrian Civil Defence, also known as the White Helmets, are signatories to the letter.

"The UN is [the] main culprit and they are as responsible as [Syrian President] Bashar al-Assad's regime," Zaher Sahloul, president of SAMS, told Al Jazeera.

Sahloul said the UN Office for the Coordination of Humanitarian Affairs, which overseas the work of NGOs on the ground, is being pressured by the Syrian government when it comes to the distribution of aid supplies and donor funds through the WoS mechanism.

"In the plan for the WoS programme, the UN removed words such as 'besieged' and allocated most of the funding to areas under government control," Sahloul said.

"The Syrian government is trying to limit funding to areas besieged by its troops ... They want to pressure them to surrender," he said.

"The UN agencies know all of these tactics. The problem is that they are not doing anything about it."

In response to the joint letter, Stephane Dujarric, the UN secretary-general’s spokesman, said in a televised statement on Thursday that the world body's operations in Syria are transparent.

"Everything is listed. We always welcome any scrutiny of our humanitarian work in Syria. Our focus is and will continue to be to deliver as much aid as possible to the Syrian people," he said.

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Al Jazeera's Shihab Rattansi, reporting from the UN headquarters in New York, said the UN admits to taking a pragmatic approach to its work in Syria.

"The UN has said it needs to deal with the Syrian government where it has control" and in "areas not controlled by the Syrian government, the UN said it does not deal with agencies connected with the regime", Rattansi said.

The mass suspension of cooperation with the UN comes after recent reports by the UK-based Guardian newspaper, which uncovered tens of millions of dollars worth of contracts awarded by the UN to the Assad regime under aid programmes.

The Syrian Arab Red Crescent (SARC), also named in the letter, is accused of operating like a ministry in the Syrian government.

"The head of SARC reports directly to the prime minister. Many believe they are as corrupt as the Syrian government," Sahloul, of SAMS, said, referring to the Red Crescent's leadership.

SARC officials could not be immediately contacted for comment.

Al Jazeera's Rattansi said it remains to be seen what impact the suspension of cooperation will have on the ground for the people of Syria.

"We should be clear here, these NGOs are not withdrawing any operational assistance to Syria. They will continue their operations. What they are doing, though, is withdrawing their cooperation with the UN in information sharing," he said.

"We have to see how significant that is in the long run. But it is clearly a shot across the bows of the United Nations.

"They say they won't cooperate with the UN on information sharing until there is an investigation into the UN's conduct in Syria and there is more oversight of UN operations in Syria."

James Denselow, a Middle East expert and research associate at the Foreign Policy Centre in London, said that Syria has become "the humanitarian challenge of a generation with various actors playing politics with humanitarian principles".

"The key question is whether this will force the UN into a new approach towards how it delivers its aid or if it sticks with the status quo, which will effectively mean even less aid for people living outside Damascus areas of control," Denselow told Al Jazeera.