The US Secretary of State John Kerry is expected to urge his Russian counterpart to ensure a 'transparent' investigation into the murder of an opponent of President Vladimir Putin.
Boris Nemtsov is believed to have been working on a report into the presence of Russian troops in Ukraine before he was shot in Moscow on Friday.
Mr Nemtsov's allies have accused the Kremlin of being behind the murder - however, President Putin has condemned the killing as 'vile' and has promised to find those responsible.
In a message to Mr Nemtsov's mother, he said: "Everything will be done so that the organisers and perpetrators of a vile and cynical murder get the punishment they deserve."
Yesterday, thousands of protesters marched through the streets of Moscow to honour Mr Nemtsov.
Organisers said more than 70,000 people joined the rally to mourn the former deputy prime minister, who was gunned down near the Kremlin last week.
Many of the demonstrators carried portraits of Putin critic Mr Nemtsov, as well as placards declaring "I am not afraid" and "He died for Russia's future".
Opposition leaders said the protest was aimed at stopping a "campaign of hate" directed at those who question President Vladimir Putin's rule.
"If we can stop the campaign of hate that's being directed at the opposition, then we have a chance to change Russia," said Gennady Gudkov, an opposition leader.
"If not then we face the prospect of mass civil conflict. The authorities are corrupt and don't allow any threats to them to emerge. Boris was uncomfortable for them."
Police estimated the crowd on Moscow's streets was around 16,000 people.
Mourners earlier laid flowers and lit candles on a bridge near the Kremlin where Mr Nemtsov was gunned down.
He was shot in the back several times just before midnight on Friday as he was walking home from a restaurant in the centre of the city.
Amnesty International said: "There is already a list of unsolved political murders and attacks in Russia. We cannot allow Boris Nemtsov to become just another name on this list."
Investigators say gunmen fired at least eight shots from a car as Mr Nemtsov walked with a woman named by Russian media as Ukrainian model Anna Duritskaya (23), who was unhurt.
Footage aired by Russian TVC channel from a distant camera showed the suspected killer running along the road and jumping into a waiting car which then sped off.
