No staff members had become ill, CNN reported.

An envelope containing an unidentified substance arrived Thursday at the U.S. Consulate in Karachi, Pakistan's largest city, spokesman Lou Fintor said.

Another envelope containing unknown matter arrived the following day at the U.S. Embassy in Islamabad, Pakistan's capital, he said.

At no time did either office close, and both will open as usual Monday, he said.

Consular and embassy staff followed proper procedures when the envelopes were received, Fintor said without elaborating.

"We take this matter very seriously, hoax or not," he told CNN.