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A US-backed alliance on Wednesday captured Syria's Tabqa and its nearby dam from the Islamic State group, a day after Washington said it would arm the force's Kurdish fighters.

The Syrian Democratic Forces, the bulk of which is made up of the Kurdish People's Protection Units (YPG), said it had "completely liberated" the Syrian city of Tabqa and the adjacent dam after weeks of fierce fighting.

"The combing operations are ongoing to ensure that the city is clear," said spokesperson Talal Sello.

Tabqa sits on the Euphrates River as well as a strategic supply route about 55 kilometres (35 miles) west of Raqqa, the Syrian heart of IS's so-called caliphate.

The UN has warned damage to the Tabqa dam - Syria's largest - could lead to massive flooding, and a source who works with the dam's technicians said they had fled the structure as fighting drew near in recent days. 

"Tomorrow (Thursday), the teams will assess the dam and the level of damage caused by the clashes," the source told AFP.

Turkey slammed the US decision to arm the YPG, which Ankara considers terrorists, while Washington sees them as an indispensable ally in the fight against IS.

The issue risks stoking tensions between the two countries less than week before US President Donald Trump and his Turkish counterpart Recep Tayyip Erdogan meet for the first time as heads of state.

'Immediately' reverse decision

Warplanes from the US-led coalition have pounded Tabqa and nearby IS positions for weeks as part of the SDF's flagship offensive for Raqqa.

The YPG said the US's "historic" decision to begin providing it with weapons and other equipment would speed up its assault on IS positions.

Spokesperson Redur Xelil said the move was "somewhat late", but would still "provide a strong impetus" to forces fighting IS.

But this sparked ire from Turkey, which regards the YPG as the Syrian branch of the Kurdistan Workers' Party (PKK), which since 1984 has waged an insurgency inside Turkey, leaving tens of thousands dead.

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"I hope very much that this mistake will be reversed immediately," said Erdogan.

"I will personally express our worries in a detailed way when we talk with President Trump on May 16," he added, saying NATO's Brussels summit on May 25 would also tackle the issue.

In a surprise announcement, the Pentagon said Trump had authorised the arming of the SDF's Kurds "to ensure a clear victory over ISIS in Raqqa".

Until Tuesday, official US policy was to supply weapons only to the Arab components of the SDF - never to its Kurds. 

The dispute over arming Syria's Kurds poisoned ties between the two NATO allies under the administration of former president Barack Obama but Ankara had hoped for smoother ties under Trump.

Prime Minister Binali Yildirim said it was "out of the question" for Turkey to accept any direct or indirect help for the PKK.

"The United States and Turkey are two major partners in NATO. We don't believe America would choose a terror group over our strategic relations," he said.

It remains to be seen what shadow the issue will cast over upcoming talks between Trump and Erdogan, touted as chance to forge a new partnership between the two sides.