John Kerry warned of "consequences" after his Russian counterpart made clear Moscow would respect the outcome of Crimea's referendum.
The nuclear agreement between Western powers and Iran was broadly welcomed by politicians, former world leaders and Middle East experts.
As Sergei Lavrov and John Kerry bid for a peaceful resolution to Syria, a former British diplomat says the US is "taking on the master".
US Secretary of State John Kerry has said Sergei Lavrov told him Vladimir Putin is not prepared to make any decision on Ukrainian's Crimea region until after Sunday's referendum.
Mr Kerry said the US and the EU are already clear on the referendum, describing it as "illegal under the Ukrainian constitution".
He said the international community will not recognise its outcome.
Mr Kerry said he also told Mr Lavrov, Russia's Foreign Minister, the US was concerned about the deployment of Russian troops in military exercises on the Ukraine border and the "increased anxiety" it had caused amid the current crisis.
The US Department of State and the Russian foreign ministry have posted pictures of John Kerry and Sergei Lavrov taking a stroll in the garden of the US Ambassador's residence in London:
Lavrov and Kerry. Hope they will be back http://t.co/ZXDFNSCzhE
Break is over. Match continues. http://t.co/C3Dmf4rq2D
David Cameron and John Kerry enjoyed a bit of a natter earlier today as they posed with William Hague for the cameras to mark the US Secretary of State's visit to Downing Street before his crisis-talks with Russia's Sergei Lavrov.
After the traditional handshake welcome, the Prime Minister described having a "lovely time" in Israel's boisterous parliament, the Knesset, a few days ago.
Mr Kerry responded that he looked forward to Mr Cameron's "download" on the key visit before joking with Mr Hague about them regularly crossing paths.
The group ended the staged sit-down by reiterating their commitment to taking action against Russia if a diplomatic solution to the Ukraine crisis cannot be found.
Secretary of State John Kerry has said the US hoped to "move forward together" with Russia "to resolve some of the differences between us" as he entered last-ditch talks over the Ukraine crisis.
Mr Kerry said he would "dig in to the issues and possibilities" with Russian Foreign Minister Sergei Lavrov during the meeting in London, which comes amid mounting international pressure for the Kremlin to help to halt Sunday's referendum in Crimea.
Their discussion comes as Germany's Bild newspaper claims the EU and US will enforce visa bans against 13 top Russian officials as sanctions if the referendum goes ahead.
Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry are due to meet in London today to discuss the situation in Ukraine.
Yesterday the pair discussed proposals for resolving the crisis in Ukraine during a telephone conversation, the Russian Foreign Ministry said.
Lavrov and Kerry talked about "the situation in Ukraine, taking into account existing Russian and U.S. proposals to normalise the atmosphere and provide for civil peace", the ministry said.
The US and the European Union will respond with a "serious series of steps" against Russia if a referendum in Ukraine's Crimea region goes ahead on Sunday, Secretary of State John Kerry warned.
Kerry told a congressional hearing he hoped to avoid such steps, which include sanctions, through discussions with his Russian counterpart, Sergei Lavrov, in London tomorrow.
He said, "If there is no sign of any capacity to be able to move forward and resolve this issue there will be a very serious series of steps in Europe and here with respect to the options that are available to us."
US Secretary of State John Kerry said there are "contingencies" in place in case Russian forces move farther east into Ukraine.
Although Kerry stressed he hopes to avoid that happening, he said there was "no telling that we can".
US Secretary of State John Kerry said tensions between Russia and the US over Ukraine "has the capacity" to affect their relationship with Moscow on Syria.
Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry discussed proposals for resolving the crisis in Ukraine during a telephone conversation on Thursday, the Russian Foreign Ministry said.
Lavrov and Kerry are due to meet in London on Friday.