The announcement comes after Secretary of State Rex Tillerson met with Turkish officials last week in Ankara, Turkey, and discussed possible strategies for defeating ISIS in Syria.
The Pentagon immediately sought to stress that it saw arming the Kurdish forces "as necessary to ensure a clear victory" in Raqqa, Islamic State's de facto capital in Syria and a hub for planning the group's attacks against the West. The arms, ammunition and vehicles will be delivered in an incremental fashion.
U.S. President Donald Trump's defence secretary emerged upbeat on Tuesday from talks with a Turkish official about the fight against IS but did not disclose any progress toward settling a row over U.S. backing for Kurdish fighters in Syria.
The YPG includes the Syrian Democratic Forces of Arab, Christian and Turkmen fighters.More news: BJP dubs Mulayam's statement on Akhilesh as 'another episode of family drama'
The equipment provided is set to include small arms, machine guns, construction equipment and armored vehicles, a USA official told CNN.
"We are keenly aware of the security concerns of our coalition partner Turkey". Bob Corker of Tennessee, said he is not anxious about how Turkey will react.
The U.S. support for the SDF has been a major strain on relations between Washington and Ankara.
Their goal, Erdogan has said, is to create a Kurdish canton that can join with Turkish Kurds of the Kurdistan Workers Party, or PKK, separatists who have been at war with the Turkish government - and which both the United States and Turkey have designated as a terrorist organization.More news: Hamas says Ismail Haniyeh chosen as Islamic group's leader
"The SDF, partnered with enabling support from US and coalition forces, are the only force on the ground that can successfully seize Raqqa in the near future", she added. "Raqqa and all liberated territory should return to the governance of local Syrian Arabs".
The Turkish president, fresh from winning the controversial April 16 referendum on enhancing his powers, has indicated that the rewards for Washington in breaking up with the YPG could be high by spurring Turkish involvement in a joint operation to take the IS fiefdom of Raqa.
Turkey carried out a series of airstrikes against the YPG last month, a move that drew immediate complaints from USA officials. In her second statement she emphasized USA solidarity with Turkey and said the US does not envision a long-term Kurdish presence in Raqqa.
When asked whether the new arms deal with the YPG could put USA forces in danger from Turkish airstrikes, Pentagon spokesman Maj. She said Mattis spoke with his Turkish counterpart, Fikri Isik, by telephone to reiterate a US commitment to "protecting our North Atlantic Treaty Organisation ally". U.S. officials have championed the fight there as a key part of the operation to retake Raqqa and an example of the group's prowess.More news: Clean air plan set to be published