US special envoy Richard Holbrooke has warned Pakistan against committing itself to a gas pipeline project with Iran because of anticipated American sanctions against Tehran.
Mr Holbrooke said Islamabad should wait until it received more details on new US legislation that could affect the multi-billion dollar project.
Iran signed a deal with Pakistan to supply it with natural gas from 2014.
Pakistan says it needs the gas from Iran to ease its growing energy crisis.
The original plan was to carry gas from Iran to Pakistan and then to India, but Delhi withdrew from the project due to differences over prices and transit fees, and also apparently due to pressure from the US.
The US Congress is preparing new legislation which will impose more sanctions on Iran because of concerns over Tehran's nuclear programme.
Mr Holbrooke, who is on a visit to Pakistan, cautioned the country against going ahead with the gas pipeline project.
"We cautioned the Pakistanis to try to see what the (congressional) legislation is before deciding how to proceed because it would be a disaster if... we had a situation develop where an agreement was reached which then triggered something under the law," he said.
He admitted that Pakistan "has an obvious major energy problem" and that the US was "very sympathetic to it".
Pakistani Foreign Minister Shah Mahmood Qureshi said that the pipeline deal with Iran did not violate existing sanctions against Iran.
Iran signed a deal with Pakistan last week to supply it with natural gas from 2014.
The pipeline was originally planned to transport gas from Iran to India through Pakistan.
The deal with India was stalled by disputes over transit fees and security issues.