On Sunday, a U.S. jet shot down a Syrian Su-22 fighter-bomber near the city of Tabqah.
A similar incident occurred June 6, and on June 8 a USA warplane shot down a pro-regime drone in Syria after it fired at coalition forces.
The US military said that it shot down a warplane that had dropped bombs near Syrian Democratic Force (SDF) fighters.
Syria's army disputed the account, saying its plane was hit while "conducting a mission against the terrorist Islamic State group".
According to a statement from the Pentagon, pro-Syrian regime forces attacked the USA -backed Syrian Democratic Forces-held town of Ja'Din, south of Tabqah in northern Syria, wounding a number of SDF fighters and driving the SDF from the town.
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As ISIS is set to be squeezed into smaller areas in Syria's east, various factions are competing over who will hold the land it leaves behind that could further complicate the conflict.
Though the two incidents were nominally unrelated, they have a common cause: the drive by Iran and Russian Federation, along with their Syrian and Iraqi Shiite clients, to dominate the space that will be left when the Islamic State is driven from its capital of Raqqa in eastern Syria, which is under assault from us -backed Kurdish and Syrian Arab forces.
"The escalation of hostilities doesn't help anybody and the Syrian regime and others in the regime need to understand that we will keep the right to self defense".
Russian Defense Ministry said its own warplanes had been in the same space as the Syrian-U.S. strike, according to The Washington Post.
The U.S. -led coalition headquarters in Iraq says a U.S. F-18 Super Hornet shot down a Syrian government SU-22 after it dropped bombs near the U.S. partner forces.
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Russia's Deputy Foreign Minister Sergei Ryabkov called the shoot down an "act of aggression".
The Dutch government is sending a military refueling plane to Kuwait to assist the US -led coalition carrying out airstrikes against the Islamic State group in Iraq and Syria.
Speaking in Washington, the top USA military officer said the two sides discussed the matter as recently as Monday morning but that further talks are needed.
It also raised questions about how U.S. President Donald Trump's administration, which had previously put Iran "on notice" for its ballistic missile tests, will respond.
They are in support of Syrian Arab and Kurdish fighters aligned against Islamic State militants.
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These Iran hawks, they said, are pushing for a Syria strategy that calls for concentrating first on defeating Islamic State, then turning on Iran and its allies, including Assad, Lebanon's Hezbollah, Iraq's Shi'ite militias, and Houthi rebels in Yemen.