The US had multiple recent incidents in which they’ve attacked Shi’ite militia forces in southern Syria, accusing them of getting too close to a rebel base which has US personnel stationed in it. The indications, however, are that the Shi’ite militias in the region have been attacking ISIS and al-Qaeda forces, not the US-backed rebels at al-Tanf.
Russia has harshly criticized the US attacks, saying that they are risking cooperation in the fight against ISIS. Syrian officials have also issued a statement harshly criticizing the US attacks, noting that the US presence was never authorized by the government in the first place, and that they consider attacks on their allies within Syrian soil is a violation of sovereignty.
The US has maintained that its various attacks were done in “self-defense” because the “regime forces” got too close to the Tanf base, despite no suggestion they were moving against the base in the first place. The US has made clear they are displeased with the Shi’ite militias’ efforts to reach the Iraqi border, and link up with Iraqi militia forces fighting ISIS there, but there appears to be no real basis for the US to prevent this move.