The International Atomic Energy Agency (IAEA) had previously reported finding "weapons grade" traces of uranium, but had not revealed that some were from uranium refined to 90 percent of the rare 235 isotope.
Iran has argued that the traces of highly enriched uranium was from contamination that occurred before imported equipment arrived in the country, the daily said.
Iranian officials, the daily added, said they could not trace the origin of the contamination since they imported their equipment from middlemen in five countries.
IAEA officials, The Washington Post said, raised the possibility the contamination may have originated in Pakistan, whose top nuclear scientist Abdul Qadeer Khan recently admitted having supplied uranium enrichment equipment to Iran and other countries.
The UN agency said it has asked Islamabad for permission to take environmental samples from enrichment facilities in Pakistan to see if the traces match those found in Iran.
"Pakistan could let Iran off the IAEA hook," a European diplomat was quoted as saying in Vienna, where the UN agency is headquartered.
US officials maintain that the weapons-grade traces in Iran provide insight to Iran's goals.
"What it shows is that they have a system that is capable of producing weapons grade uranium," said an unnamed US official speaking in Washington.
"If it's an assembly that was removed from Pakistan or elsewhere, it's already battle tested," he said.
Iran on Wednesday criticized European states for bowing to US pressure to condemn Tehran's atomic program before the UN nuclear watchdog and threatened to cut cooperation with the IAEA.
Iranian ambassador Pirooz Hosseini told reporters at a meeting of the International Atomic Energy Agency (IAEA).
The IAEA's 35-nation board of governors was Thursday still debating a resolution on Iran, with a vote expected later in the week.
A draft resolution lists Iranian failures to report sensitive nuclear activities, despite Tehran's claim to have fully disclosed its nuclear program in a declaration to the IAEA last October.
But the draft resolution puts off any immediate reaction, such as declaring Iran to be in non-compliance with the international nuclear Non-Proliferation Treaty (NPT), a move that would mean the issue would be taken up by the UN Security Council which could pave the way towards possible sanctions.