"He will leave this afternoon for Brussels," a diplomatic source said.
Foreign Minister Jose Manuel Garcia-Margallo, who arrived in Tehran on Saturday for a planned four-day visit, is to attend an emergency meeting of EU foreign ministers on Monday.
"We are concerned about the territorial integrity of Ukraine," Garcia-Margallo said in a joint press conference with his Iranian counterpart Mohammad Javad Zarif
Zarif said Iran too was "concerned about the developments in Ukraine."
"We hope there will be a peaceful settlement to the crisis," he added.
The EU session was called after the Russian parliament approved President Vladimir Putin's request for sending troops into neighbouring Ukraine's Crimea region, triggering an international outcry.
US President Barack Obama warned Moscow on Friday that there would be "costs" if Russia intervenes militarily while German Chancellor Angela Merkel has said she is 'worried' about events in the Crimea.
Under the threat of a Russian intervention, the Western-backed interim government in Kiev has put the military on full combat alert.
It also asked the US-led NATO alliance to help defend its "territorial integrity and sovereignty."
In Tehran, the foreign policy adviser to Iran's supreme leader Ayatollah Ali Khamenei appeared to blame NATO for the crisis.
"NATO is greedily eyeing Ukraine under the pretext of it joining the military alliance," Ali Akbar Velayati told the Mehr news agency.
"We hope their interference is terminated."
NATO has called for emergency talks Sunday in Brussels on the "grave situation" in Ukraine.
NATO set up a joint commission with Ukraine in 1997 to oversee relations, and in 2008 agreed that Kiev could eventually be considered for membership of the Cold War-era alliance.