"This cannot be the way in the 21st century to conduct international affairs," Hague said in Kiev, two days after Russian President Vladimir Putin won senators' authorisation to send troops into Ukraine.
"It is not an acceptable way to behave and there will be consequences and costs," he added.
Hague did not spell out what these consequences might be, but insisted that Britain was discussing economic and diplomatic options, not military ones.
His comments mirrored earlier ones by US President Barack Obama, who warned Putin at the weekend that Moscow's actions in Ukraine would carry "costs".