The move will help Kurdish fighters battling IS for Raqqa but will alarm neighbouring Turkey.
US President Donald Trump has approved supplying weapons to Kurdish forces fighting so-called Islamic State (IS) in Syria, the Pentagon says.
Kurdish elements of the Syrian Democratic Forces (SDF) would be equipped to help drive IS from its stronghold, Raqqa, a spokeswoman said.
The US was “keenly aware” of Turkey’s concerns about such a move, she added.
Turkey views the Kurdish rebels as terrorists and wants to stop them taking more territory in Syria.
The SDF, which comprises Kurdish and Arab militias, is already being supported by elite US forces and air strikes from a US-led coalition.
The group is currently battling IS for control of the city of Tabqa, an IS command centre just 50km (30 miles) from Raqqa.
A Pentagon source told the BBC the equipment would include ammunition, small arms, machine guns, heavy machine guns, construction equipment such as bulldozers and armoured vehicles.
The source added that the US would “seek to recover” the equipment afterwards.
The US believes the Kurdish fighters – known as the YPG – will be essential to the city’s downfall.
The Pentagon sees them as the most disciplined and organised of the anti-IS groups but Turkish opposition has meant Washington has had to tread a fine line.
The imminence of the fight for Raqqa means delay is no longer an option and the Kurds will be getting a range of equipment.
US sources say they have received assurances from the Kurds that they will leave Raqqa to be governed by Syrian Arabs after the battle.
Turkey’s President Erdogan is due in Washington next week – he will not be a happy visitor.
No timeline has been given for when the weapons would start to be supplied.
“We are keenly aware of the security concerns of our coalition partner Turkey, ” said Pentagon spokeswoman Dana White, who is travelling with US Defence Secretary Jim Mattis in Lithuania.
“We want to reassure the people and government of Turkey that the US is committed to preventing additional security risks and protecting our Nato ally.”
Turkey sees the Syrian Kurds as an extension of the PKK separatist group inside Turkey which has been designated a terrorist organisation by the US and EU.
