Ukraine's acting president asked for help from regional governors to stop upheaval in the east from spreading, as pro-Russia militants seized buildings in Horlivka, a city near Donetsk.
President Obama said the measures are aimed at 'changing Putin's calculus' on Ukraine, and that further 'Russian aggression' could trigger more sanctions.
The interim government said it had 'obtained US support' for operations against pro-Russian militias, which it restarted after saying that a local politician was found dead and tortured.
Less than 24 hours after it was signed by officials from Russia, Ukraine, the US, and the EU, the ambitious roadmap to defuse Ukrainian tensions was showing cracks.
During his annual public call-in show, the Russian president said he would send troops into Ukraine to 'protect' locals if necessary.
Kiev says it has begun operations to displace pro-Russia protesters holding buildings in the country's east, though there have been few signs of activity so far.
Russia continues to deny NATO accusations that it is gathering troops on the border as it hastens to prepare for a possible cutoff of essential Ukrainian-made military parts.
NATO's top military commander says that he will present a package of measures next week laying out how the alliance will respond to the Russian buildup along Ukraine's borders.
Kiev gave protesters in the east 48 hours to come to the table. But in both Donetsk and Luhansk, armed demonstrators remained behind their barricades.