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".
The four-way talks between diplomats from the US, Russia, Ukraine and the EU have now run an hour over schedule, according to ITV News Diplomatic Correspondent John Ray.
Geneva Ukraine talks now an hour over schedule. A tentative sign of progress?
Being told lots of serious faces among ministers as they take breaks between rounds of one on ones. Sounds like a good thing....
One source says no word yet on agreed statement but that they are working on one is way ahead of where some feared they'd end up today
US Secretary of State John Kerry has met his Ukrainian and Russian counterparts ahead of four-way talks between representatives of the US, Ukraine, Russia and the EU.
Mr Kerry met Russia's Sergei Lavrov and Ukraine's Andriy Deshchytsia, having earlier held bilateral talks with EU foreign affairs chief Baroness Ashton.
US Secretary of State John Kerry has begun bilateral talks with the EU's high representative for foreign affairs, Baroness Ashton, about the situation in Ukraine.
Later this morning the pair will join the Russian and Ukrainian foreign ministers for a further round of talks on resolving the crisis.
US Secretary of State John Kerry tweeted on the first anniversary of the Boston Marathon bombings:
One year later, even if you were miles from home, no Bostonian will forget this day. Remember victims, honor heroes. We are #BostonStrong.
Russia and the United States exchanged strong-worded warnings over armed actions in Ukraine today.
Russian Foreign Minister Sergei Lavrov said that any armed action by Ukrainian authorities in the east of the country would undermine efforts at a diplomatic solution to the conflict.
Mr Lavrov made the warning during a call from US Secretary of State John Kerry, who in turn expressed concern about Russia's role in "inciting" trouble in eastern Ukraine, the Russian Foreign Ministry said in a statement.
The White House warned Russia against further military action in Ukraine after armed separatists seized government offices in the eastern city of Slaviansk today, saying the situation was reminiscent of what recently happened in Crimea.
Mr Lavrov said Ukraine was "demonstrating its inability to take responsibility for the fate of the country."He said that any use of force against Russian speakers in the east of Ukraine "would undermine the potential for cooperation."
US secretary of state John Kerry is heading back to the Middle East for talks with Israeli and Palestinian leaders as peace negotiations hit a critical make-or-break point.
The US state department said Mr Kerry will travel to Israel and the Palestinian territories amid a flurry of diplomatic activity by American mediators, in the hope of salvaging the troubled negotiations and getting them to extend the talks beyond a current late-April deadline.
The latest Ukraine crisis talks between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov in Paris have ended after four hours, a US official has said.
US Secretary of State John Kerry has said he hopes the current situation in Crimea will not have an impact on cooperation with Russia over the ongoing destruction of Syria's chemical weapons.
Mr Kerry also said the US is "convinced" that the Syria could move faster on the destruction programme than it is currently.
US Secretary of State John Kerry has told his Russian counterpart, Sergei Lavrov, that Russia must pull back its troops to their bases in Crimea, a US official has said.
Mr Kerry also reiterated that the White House regards today's referendum on the future of Crimea as illegal and the US government will not recognise the outcome.
Two of the EU's most senior officials, Jose Manuel Barroso and Herman van Rompuy, also released a joint statement calling the referendum "illegal and illegitimate".