UN humanitarian chief Valerie Amos urged Tehran at the end of a visit on Sunday to use its influence with ally Syria to allow greater aid access to the country.
"I asked the foreign minister (Mohammad Javad Zarif) to use Iran's influence with the (Syrian) government to continue to press for greater access for humanitarian organisations," she told a news conference.
"I did not specify areas because it is important that humanitarian organisations are allowed to go where people are in the greatest need."
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She said: "Getting to some areas is very difficult because of security, and the proliferation of groups operating on the ground means that you have to negotiate with a large number of actors."
Amos said that the humanitarian situation in Syria was "dire," with "6.8 million people in urgent need of help, nearly a third of the population" in the conflict-wracked country.
"We are extremely concerned about the rising level of brutality inside Syria," she said.
Amos said Iran was among countries which "have a major role to play in terms of the relationships they have either with the government or the opposition, to help to get us the access we need."
The United Nations aims to feed three million Syrians but has so far received only 40 percent of the $4.4 billion it has requested to finance humanitarian operations, she said.