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After the failed attempt to enter the ISIS-controlled city of Manbij, the Syrian Democratic Forces (the Kurdish YPG and some Arab units) have launched a military operation to the west from Manbij.
SDF units, supported by the US-led coalition air powerm have seized the villages Sayadah and Zunqul and launched an advance in the outskirts of the junction of Arab Hassan Kabir. Separately, ISIS militants have conducted a counter-attack in the direction of Ar Rus.
The situation in the area of Manbij could be considered as a stable. Kurdish units, supported by US warplanes, are gaining the ground in the area. However, they are not able to seize a major city with predominantly Suni Arab population, yet.
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Strategically, this is sound. Taking ground in the west limits the possibility for IS counterattacks, liberates the heavily kurdish areas and, if they take the whole kurdish belt, makes for a connection to Afrin in the Ahres area.
They might need extra troops, but reinforcement from Kobane is now easy.
As for Manbij and maybe later Al Bab, go slowly and carefully. Better a week extra in fighting then heavy casualties among civilians and fighters.