American-backed Iraqi forces have been unable to cut off the Islamic State's use of the Euphrates River as a major supply line into Ramadi, allowing the militants to continue bringing weapons and people into the city on boats and barges, a defense official said.

"We have not been able to cut the Euphrates River yet," Army Col. Steve Warren, a Defense Department spokesman in Baghdad, said Wednesday.

The river connects the Islamic State militants to their stronghold in Syria and is emerging as a focus of the Iraqis' nearly six-month effort to retake the city.

Iraqi forces have surrounded most of the city, but have not yet seized the Palestine Bridge, a primary crossing over the Euphrates on the city's northwestern side.

"When our friendly forces are able to seize that bridge, they will be able to then cut the Euphrates River, which will substantially, significantly constrict ISIL's ability to resupply itself," Warren said, using an alternative acronym to refer to the militant group.

"ISIL, of course, knows that. That's the primary line of communication. And so they're defending it fiercely. So this fight is going to continue and it's going to continue as long as it takes to get it done," Warren said.

Islamic State militants seized control of Ramadi in May and the Iraqi security forces' counter-assault has been slow. The Iraqis spent months conducting "shaping" operations followed by several months in the "isolation" phase.

Several weeks ago, Warren told reporters that it was time for the Iraqis to mount a direct assault on the city.

"Iraqi ground forces recently trained and equipped by the coalition have been deployed around Ramadi in time for the decisive phase of this operation. We now believe that battlefield conditions are set for the ISF to push into the city," Warren said Oct. 13.

But Iraqi forces have encountered roadside bombs, snipers and other lethal obstacles, resulting in only incremental progress.

"The enemy here has had months to prepare complex defenses inside the urban center of the city. And that is going to be a difficult, hot, dangerous, scary fight. And it will take — it will take a while," Warren said Wednesday

The river is a hub of civilian and commercial maritime traffic, making it difficult for the U.S. to clearly identify the boats or barges moving military supplies into the city, Warren said.

U.S. airstrikes sometimes target Islamic State barges along the river, but the U.S. is also cautious to prevent civilian casualties, he said.

"It's a water-borne highway. So you can't, obviously, you're not going to strike every single vessel that we see," Warren said.