He was speaking after reports that the rebels had extended the area they control, violating a ceasefire plan.
Ukraine says Russia has more than 9,000 soldiers fighting alongside the rebels, a claim it denies.
Meanwhile, foreign ministers from Ukraine, Russia, France and Germany issued a joint call to end the fighting, following talks in Berlin.
Speaking after the meeting, German Foreign Minister Frank-Walter Steinmeier said it was not a breakthrough "but I think we saw tangible progress".
He also said they had agreed on a procedure for pulling back heavy weapons 15km (nine miles) from a demarcation line defined in last year's Minsk agreement.
Mr Kerry said the recent upsurge in fighting was "an alarming situation" adding that the US was "particularly concerned" by rebel moves to "attempt to gain control of a very significant rail juncture" in eastern Ukraine.
He said there had been a large extension of the line of control that separates rebel-held territory from the rest of Ukraine.
"This is a blatant land grab and is in direct contravention to the Minsk [ceasefire] agreements which they signed up to," he added.
The US ambassador to the UN, Samantha Power, accused Russia of escalating the violence.'