Hague arrived in Brussels for a meeting with foreign affairs ministers from the European Union to discuss the crisis in Ukraine following a weekend referendum in the Crimean peninsula to join Russia.
Hague said that, with no campaigning or other processes, the vote was illegitimate and not recognized by members of the EU.
Russian President Vladimir Putin said Monday the referendum was in line with international law, state-supported news agency RIA Novosti reported.
President of the European Council Herman Van Rompuy and President of the European Commission Jose Manuel Barroso issued a statement Sunday saying the solution to the crisis in Ukraine lies in territorial integrity and respect for its sovereignty. Russia, they said, is called on to withdraw its forces from the region.
Hague said members of the EU would consider asset freezes and travel bans for this interfering with Ukraine's sovereignty.
"We are also prepared to move to further measures and there will be long-term costs and consequences for Russia if they continue to approach things in this way," he said Monday.
Hague said military action has no place in 21st century diplomacy, a sentiment echoed by Van Rompuy and Barroso.
"Only working together through diplomatic processes, including direct discussions between the governments of Ukraine and Russia, can we find a solution to the crisis," they said.