LONDON: Iran's oil exports may struggle to rebound after a sharp drop  in December, a leading tanker-tracking company said, due to the prospect of less generous sanctions waivers from Washington.

Geneva-based Petro-Logistics said on its website that Iranian crude and condensate exports in December "fell steeply" from November to less than 1 million barrels per day (bpd) due to U.S. sanctions - lower than some other estimates.

While there is scope for buyers to boost exports under existing U.S. sanctions waivers, this period ends in May 2019 and the United States has said a tougher waiver process may be put in place at that point, Petro-Logistics said.

"It is likely that buyers may not want to risk increasing their dependence on Iranian volumes in the intervening period, even if they are currently allowed to do so," the company said.

"Having worked hard to organise alternative sources of oil to replace Iranian volumes, buyers may be unwilling to back out from these arrangements."

Iranian shipments have become more opaque since the sanctions took effect, leading to a widening range of views on how much oil the OPEC member is sending to the global market.

Kpler, another firm that tracks Iranian flows, put Iranian crude and condensate exports at 1.35 million bpd in December.