The weapons are being delivered for use by Kurdish fighters ahead of the upcoming offensive to recapture Raqa, the last major bastion for IS in Syria and the capital of their supposed "caliphate."
President Donald Trump on Monday "authorised the Department of Defence to equip Kurdish elements of the Syrian Democratic Forces as necessary to ensure a clear victory over ISIS in Raqa," Pentagon spokeswoman Dana White said in a statement.
"The SDF, partnered with enabling support from US and coalition forces, are the only force on the ground that can successfully seize Raqa in the near future."
The Kurdish elements of the SDF are from the Kurdish Peoples' Protection Units (YPG) and they have been the main faction fighting IS on the ground in Syria.
But Turkey says YPG fighters are linked to Kurdistan Workers' Party (PKK) separatists inside Turkey, who have waged an insurgency since 1984 that has killed over 40,000 people.
Turkish war planes carried out strikes on YPG forces in Syria on April 27 and also hit Kurdish forces in neighbouring Iraq in what Ankara described as "terrorist havens".
Turkey's concerns
Tuesday's announcement comes ahead of Turkish President Recep Tayyip Erdogan's visit to Washington next week to meet with Trump.
Charles Lister, a senior fellow at the Middle East Institute, said it is unclear how Washington can soothe Turkey's concerns over the arming of the Kurds.
He noted that the US government's National Counter-Terrorism Center previously labelled the YPG as the PKK's Syrian affiliate, but scrapped that description once the US began working with them in late-2014.
"There really cannot be any ignoring the fact that the YPG is the official affiliate of a terrorist organization that Turkey has been fighting for over 30 years," Lister told AFP.
"We have many reasons to be very frustrated with the Turks, but Ankara has a justified reason for being infuriated by our support for the YPG."
Pentagon chief Jim Mattis, who arrived in Vilnius late Tuesday as part of a Europe trip, earlier attended a summit in Copenhagen for senior leaders from the top 15 countries in the anti-IS coalition, including Turkey.
Though Mattis met with Turkish officials, he did not tell them of the decision to arm the Kurds, a US official said.
Still, Mattis gave a positive assessment of the role Turkey will play in the lead up to the Raqa fight.
"Our intent is to work with the Turks, alongside one another to take Raqa down," Mattis said.
Anti-IS coalition
The US-led coalition fighting IS in Iraq and Syria discussed the campaign's next steps as the jihadists' "caliphate" around Raqa is collapsing.
Though officials warn that military action will continue for some time, they are generally upbeat about the progress and quickening momentum of the fight.
"We examined the enemy situation and discussed the next steps to make sure we are all on the same sheet of music. We are going to further accelerate this fight," Mattis said.
After months of brutal, street-by-street combat, IS has now lost control of most of its stronghold of Mosul in Iraq, while the jihadists have become largely isolated in Raqa.
Several coalition countries are keeping a nervous eye on the region as IS-held territory diminishes.
Thousands of foreign fighters remain in Iraq and Syria, and coalition nations -- particularly in Europe -- are bracing for a possible wave of battle-hardened jihadists returning home.
According to a senior US administration official, Interpol has identified 14,000 foreign fighters it knows have travelled to Syria and are still alive.
The campaign against IS began in autumn 2014 and has seen the Iraqi security forces -– backed with coalition training and air power –- reverse humiliating losses and recapture several key cities including Ramadi and Fallujah.
Iraq's second city Mosul is now mainly back under Iraqi control, though IS continues to hold the Old City on the west side, where its fighters are preparing for a bloody last stand.
Trump came to power on a pledge to destroy IS.
Though much of the groundwork had already been laid and the coalition had conducted thousands of strikes, US military leaders credit him with delegating them greater authority, enabling a quickening pace of operations.
But critics say the additional strikes have accelerated the rate of civilian deaths.
