The United States has delivered a fresh supply of ammunition to Syrian Arab fighters ahead of an expected stiff battle with Islamic State as they push toward the Syrian town of al-Shadadi, a key logistics hub for the group, said US officials.

The munitions were shipped into Syria over land in recent days to Syrian Arab forces fighting in the northeast part of the country, said the officials, who spoke on condition of anonymity due to the sensitive nature of the operation. It appeared to be the third delivery of ammunition to the Syrian Arabs since the United States started supplying them with an airdrop in October.

The Syrian Arabs are allied with Kurdish fighters, and the initial shipment of US ammunition unnerved NATO ally Turkey, which is sensitive to any operations that could benefit Syrian Kurdish YPG militia.

The Syrian Arabs number around 5,000 fighters. With the Kurds and others, they form the so-called Syrian Democratic Forces seeking to claw back land from Islamic State, officials say.

The US officials said the fighters were preparing eventually to move toward al-Shadadi, which is located on a strategic network of highways. Capturing it would help isolate Raqqa, Islamic State's defacto capital.

US Army Colonel Steve Warren, a Baghdad-based spokesman for the US-led coalition against Islamic State, said the militants used al-Shadadi to stage weapons, equipment and personnel for distribution throughout the battlefield.

Warren declined to comment on any specific US resupply operations but noted past US commitments to carry them out.

The Pentagon also declined to comment on any specific operations but noted President Barack Obama has said the support of Syrian forces on the ground is a key part of his strategy for combating Islamic State.

BATTLEFIELD ADVANCES

US officials, speaking on condition of anonymity, said they expect Islamic State to put up a tough fight for al-Shadadi, largely because of its strategic importance.

One official said the group was believed to be digging long trenches and berms to prepare fighting positions.

Washington's strategy in Syria has shifted this year from trying to train thousands of vetted fighters outside the country to supplying groups headed by US-vetted commanders.

In October, Obama decided to deploy dozens of special operations forces to northern Syria to coordinate with local ground forces, acting in an advisory role away from combat.

Speaking at the Pentagon on Monday, Obama said the special forces had already begun supporting local Syrian forces as they push south.

The US military has said it would dole out the ammunition to Syrian Arabs as the fighters showed progress on the battlefield pushing into Islamic State-held territory.

That began in earnest with the capture of the town of al-Hawl after the October airdrop and has been followed by the seizure of smaller villages further south this month.

The US military estimates the larger Democratic Forces of Syria has captured around 1,000 square kilometres of terrain in the past six weeks or so, bolstered by coalition air strikes.

US officials declined to estimate how long it might take for the Syrian Arabs and others who form part of the Syrian Democratic Forces to take al-Shadadi.

The battle against Islamic State has often confounded US expectations, sometimes moving slower than hoped. The Iraqi forces have fought for months to retake the city of Ramadi, which fell to Islamic State in May.

But the coalition's Kurdish allies needed only 48 hours to claim the town of Sinjar in Iraq last month, cutting off a key Islamic State supply route.