WASHINGTON (Reuters) - The United States warned Russia on Friday that its continued support of separatists fighting in Ukraine despite a cease-fire agreement was a direct threat to the "modern global order" and could bring additional costs.
It was the strongest U.S. warning to Russia since the violation of a European-brokered truce last Sunday and followed calls from the U.S. Congress for new sanctions on Moscow and to provide weapons to Kiev.
"Russia's continued support of ongoing separatist attacks ... is undermining international diplomacy and multilateral institutions - the foundations of our modern global order," U.S. State Department spokeswoman Jen Psaki said.
She said Russia must honor its commitments immediately.
Ukraine on Friday accused Russia of sending more tanks and troops into Ukraine and said they were heading toward the rebel-held town of Novoazovsk, 40 km (25 miles) east of the port city of Mariupol, on what could be the next battlefront.
Despite its warning Washington said it still believed that diplomacy was the best way to end the 10-month-old war, although Psaki acknowledged that the United States was concerned with fresh attacks near Mariupol.
"If Russia and the separatists fail to implement the agreement, end the violence ... there will be additional costs," Psaki told a regular briefing.
A group of senior senators, including Democrat Richard Durbin and Republican John McCain, sent a letter to U.S. Secretary of State John Kerry saying international sanctions on Russia should be tightened immediately.
They said Russian President Vladimir Putin wanted to capture more Ukrainian territory and it was time to supply arms to the Kiev government.
"We can only conclude that in the absence of greater Western support for Ukrainian forces, a Russian-imposed military outcome will continue to unfold in Ukraine," the lawmakers wrote.
The Obama administration has said it is looking at ways to further support Ukraine, including heavier sanctions on Russia and possible arms for Ukraine, although officials stress the priority is to find a political solution.
(Reporting by Lesley Wroughton; Writing by Susan Heavey; Editing by Doina Chiacu and Lisa Shumaker)