Speaking before a meeting in Luxembourg of European Union foreign ministers, Hague said leaders of the bloc's 28 states would weigh Russian actions at a summit in Brussels on Friday.
The EU has drawn up plans to impose a range of economic penalties on Russia, but has held back from imposing them because of concerns among some member states about antagonising their major energy supplier.
"(Russian president Vladimir) Putin should be in no doubt that in the EU we are ready to take those measures," Hague told reporters.
"By Friday, we will be able to see how Russia is responding to the peace plan."
The United States has already threatened sanctions on Russia's financial, defence and hi-tech industries, as more Russian military material has flowed into Ukraine.
"The idea here is to deny Russia the kinds of investment and next-generation technology that it needs to continue to grow," a senior US administration official said last week.
The EU has so far imposed several rounds of sanctions, focusing largely on targeting individuals accused of destabilising Ukraine and involved in Moscow's annexation of Crimea.
Also in Luxembourg, EU foreign policy chief Catherine Ashton said sanctions would be on the agenda this week, but stopped short of saying if she supported any new measures.
Sweden's Foreign Minister Carl Bildt, said there were various points of view within the EU, but the option of tougher sanctions remained on the table.
Ukraine President Petro Poroshenko wants Putin's unqualified backing for a 15-point peace plan.
This will include the signing on Friday of an association agreement with the EU which contains a free-trade deal as well as a series of talks with separatist rebels and a seven-day unilateral ceasefire.