Russia has an obligation to ensure that pro-Moscow forces comply with the terms of the international deal aimed at calming tensions in Ukraine, William Hague said.
The Foreign Secretary warned that sanctions were still an option unless there was evidence of Russian efforts to de-escalate the crisis within the next few days.
Hopes of progress following the diplomatic deal suffered a setback as p ro-Russian insurgents in Ukraine's east said they will only leave occupied government buildings if the interim administration in Kiev resigns.
Denis Pushilin, a spokesman of the self-appointed Donetsk People's Republic, told reporters that the insurgents do not recognise the Ukrainian government as legitimate.
Mr Hague said the government in Kiev had begun work on meeting the conditions of the agreement reached between Ukraine, Russia, the EU and US in Geneva and called on Vladimir Putin's administration in Moscow to do the same.
"We do want to see, over this weekend, Russia to take the necessary actions to reduce tensions, to make sure the agreement at Geneva yesterday is upheld," he told Sky News.
"We believe that Russia contributed to destabilising the eastern Ukraine over the last week, now it is an important obligation on them to contribute to stabilising it.
"We will all want to see evidence of that, otherwise we will return to imposing more sanctions on Russia, as we agreed at the beginning of the week."
He said the Geneva deal was an opportunity for "everybody involved to step back from the brink".
"We should welcome that agreement but stress the need for implementation," he said.
"This doesn't mean the Ukraine crisis is over; it does perhaps mean that the crisis is entering a new phase."
Mr Hague told the BBC that Moscow had a "great deal of influence" over the opposition forces in eastern Ukraine and " we are absolutely sure that Russia took part in what happened over the last 10 days - the arms, the equipment, the training, the co-ordination".
He added: "There have been denials, but of course there were Russian denials at the beginning that it was Russian forces involved in the Crimea and now it's openly acknowledged by president Putin.
"So we are convinced of that and since Russia contributed so much to destabilising eastern Ukraine over the last week, they can contribute to stabilising it and so we look to them to do so."
The agreement, reached after seven hours of negotiation in Geneva, requires all sides to refrain from violence, intimidation or provocative actions.
It calls for the disarming of all illegally armed groups and for control of buildings seized by pro-Russian separatists during the protests to be turned back over to authorities.
It also gives amnesty to protesters who comply with the demands, except those found guilty of committing capital crimes.
Monitors from the Organisation of Security and Co-operation in Europe (OSCE) will be tasked with helping Ukraine authorities and local communities comply with the requirements in the agreement.
Downing Street said the UK is to provide an extra £1 million to support the expansion of the OSCE's monitoring mission.
David Cameron spoke to US president Barack Obama by telephone to discuss the situation.
Number 10 said: "They agreed that the outcome of the Geneva meeting represented a positive step forward, but that it was essential that the agreements reached were rapidly implemented in order to reduce tensions and create space for political dialogue in Ukraine.
"They agreed that the United States and European Union should do all they could to help implement these agreements, in particular through providing additional support to the OSCE special monitoring mission."