Foreign Secretary Philip Hammond has accused Russian President Vladimir Putin of acting like a "mid-20th century tyrant" over Ukraine,
Mr Hammond said that President Putin reverse its annexation of the Crimea and start respecting international law.
He also predicted that Russian economic decline - partly as a result of international sanctions - would curb its "outrageous" foreign adventures.
A claim that Russian President Vladimir Putin has Asperger's syndrome is "stupidity not worthy of comment," a Kremlin spokesman said.
Dmitry Peskov's comment to the Gazeta.ru website comes after it was revealed that a Pentagon report, written in 2008, theorised the Russian president has "an autistic disorder which affects all of his decisions".
The study, conducted for Pentagon thinktank the Office of Net Assessment, monitored Putin's movements and concluded that in very early life he "suffered a huge hemispheric event".
White House spokesman Josh Earnest said he had no comment on the report.
People in the Russian capital of Moscow have been seen celebrating president Vladimir Putin's 62nd birthday today.
A huge banner hung from a bridge in the city showing US president Barack Obama wearing a T-shirt wishing happy birthday to 'global daddy' Putin.
Shops in the capital sold T-shirts and sweatshirts emblazoned with their leader's face.
But demonstrators in Poland chose today to protest against Putin's rule outside the Russian embassy in Warsaw.
A deal to end fighting in eastern Ukraine could be reached this week, Russian President Vladimir Putin has said.
Following a phone call with Ukrainian President Petro Poroshenko, Putin said their views on ending the violence were "very close" and suggested Kiev and the pro-Russian separatists could reach an agreement by Friday.
Earlier US President Barack Obama said a ceasefire would only succeed if Russia stopped sending troops "disguised as separatists" into Ukraine.
Russian President Vladimir Putin and his Ukrainian counterpart Petro Poroshenko have spoken by phone and "largely agree" on a possible way out of the Ukraine crisis, Interfax news agency reported, citing Putin's spokesman.
"The viewpoints of the presidents of the two countries largely coincide on possible ways out of the grave crisis situation," Dmitry Peskov was quoted as saying.
President Vladimir Putin said Moscow could not stand aside when people were being shot at in Ukraine, he told Russia's state TV Channel 1 in an interview.
It must be borne in mind that Russia cannot stand aside when people are being shot at almost at point blank.
Mr Putin also said ending the crisis was down to "the political will of the current Ukrainian authorities".
The West has accused Moscow of supporting pro-Russian separatists who are fighting Ukrainian government forces in the east of the country.
Russia president Vladimir Putin said it is impossible to say when the Ukraine crisis will end and it depended on the political will of Ukraine's leadership, according to Reuters citing RIA news agency.
President Vladimir Putin has said Russia will continue to provide humanitarian aid to civilians in eastern Ukraine, the Kremlin announced.
He is also reported to have ordered separatist rebels to provide a humanitarian corridor for encircled Ukrainian troops to exit.
Russian President Vladimir Putin has reportedly told German Chancellor Angela Merkel that he decided to send a humanitarian aid convoy into Ukraine because he could no longer wait.
According to the Kremlin account of the phone call, President Putin expressed "serious concern" about the military escalation in eastern Ukraine.
Russian President Vladimir Putin has told his government to prepare for retaliatory measures against the latest western sanctions, the Tass News Agency reports.
"Of course, it should be done carefully in order to support domestic producers and not hamper consumers," he was quoted as saying.