The official reason for the cancellation was the ash cloud from an Icelandic volcano that has caused air travel chaos across Europe, as announced by a U.N. spokesman.
But several diplomats said on condition of anonymity that intensifying talks on a fourth round of sanctions against Iran over its nuclear program also played a role.
"The Americans are very keen to get a resolution finished this month," said one diplomat familiar with the negotiations. "It's no coincidence that the (six) are meeting again Monday. It was a consideration in the decision to cancel the trip."
Diplomats from the five permanent Security Council members — the United States, Britain, France, China and Russia — and Germany are meeting almost daily as they struggle to agree on what punitive measures could be included in a resolution to put to the 15-nation Security Council.
The six envoys have been discussing a U.S. draft proposal, first circulated weeks ago, that provides for a fourth round of sanctions on Iran for its refusal to stop uranium enrichment. The West accuses Tehran of seeking to produce atomic arms but Tehran says it aims only to generate electricity.
The U.S. draft proposes new curbs on Iranian banking, a full arms embargo, tougher measures against Iranian shipping, moves against members of Iran's Islamic Revolutionary Guard Corps and firms they control and a ban on new investments in Iran's energy sector.
Western diplomats familiar with the talks said they are far from an agreement and the negotiations could drag on until June. The Chinese, and to a lesser extent the Russians, are pushing the Americans and Europeans to soften the draft.