Chinese companies banned from doing business in the United States for selling military equipment to Iran are evading US sanctions, The Wall Street Journal said on its website citing a nonproliferation watchdog group.

The Chinese firms are continuing to do brisk trade with US companies and the indications are that US sanctions have become so numerous and complex that they have become difficult to enforce, the Wisconsin Project on Nuclear Arms Control said.

"We spend a lot of time convincing other countries that we need tighter sanctions on Iran when we need to better enforce our own laws already on the books," says Wisconsin Project director Gary Milhollin, a former Pentagon consultant on nuclear-proliferation matters.

The sanctions breach by Chinese firms comes at a time when President Barack Obama considers a fresh batch of sanctions against Iran for harboring suspected nuclear weapons ambitions.

The Wall Street Journal said it consulted some US companies who said they were unaware they were doing business with banned entities.

The Treasury Department''s Office of Foreign Assets Control, tasked with policing the sanctions regime, has not fined any US companies for trading with Chinese companies banned in 2006, The Wall Street Journal said.