Russia, Ukraine and the West reached a surprise deal Thursday aimed at easing the worst East-West crisis since the Cold War, but US President Barack Obama cautioned it was uncertain if Moscow would stand by the agreement.
The agreement laid out concrete steps to "restore security for all citizens" and crucially urged "all illegal armed groups" to disarm and vacate "seized buildings".
While not spelt out explicitly, that was a likely reference to pro-Kremlin separatists who have taken over parts of Ukraine's restive southeast.
The deal appeared to mark a sharp change from the tone taken by Russian President Vladimir Putin earlier in the day, when he left the door open for armed intervention in Ukraine.
UN chief Ban Ki-moon said he was encouraged that the four-way talks yielded a deal, and said he expected all sides to "show their serious intention" to implement the agreement.
German Foreign Minister Frank-Walter Steinmeier also welcomed it as a "first important step", but Obama gave a cautious reaction to the agreement, saying: "We are not going to count on it until we see it."
"I don't think we can be sure of anything at this point. I think there is the possibility, the prospect, that the diplomacy may de-escalate the situation," he told reporters.
At the same time, Obama said he was coordinating with leaders in Europe about further sanctions against Moscow if progress was not evident within days.
"We have put in place additional consequences that we can impose on the Russians if we do not see actual improvement of the situation."
The United States and European Union have already imposed punitive sanctions on key Russian and Ukrainian political and business officials, including members of Putin's inner circle.
But wider-reaching sanctions could hit Russia's key energy sector, dealing a blow to an already struggling economy.
- Plunging into an 'abyss' -
Earlier Thursday, Putin said he hoped he would not have to use his "right" to send Russian troops into its western neighbour, in what appeared to be a thinly-veiled threat against Kiev.
"I very much hope that I am not obliged to use this right and that through political and diplomatic means we can solve all the acute problems in Ukraine," he said, warning that Ukraine was plunging into the "abyss".
In an escalation of tensions, Kiev banned all Russian males aged 16 to 60 from entering Ukrainian territory, a measure described by Russia's Foreign Minister Sergei Lavrov as "disgusting".
Kiev's new, untested leaders earlier this week launched a much-hyped military operation against separatists in the restive east, but the action failed spectacularly, with the militants blocking troops and even seizing six of their armoured vehicles.
Washington and Kiev have accused Russia of supporting these insurgents, who have been behaving in very similar ways to militants who seized public buildings in the Crimean peninsula before it was annexed by Moscow last month.
But Moscow has categorically denied links to the gunmen and blamed Ukraine's interim leaders -- brought to power in February after the ouster of pro-Kremlin president Viktor Yanukovych -- of bringing the country close to civil war.
- 'Don't think it's over' -
The upper house of parliament on March 1 authorised the Russian leader to send troops into Ukraine after Yanukovych's ouster, and Moscow later went on to take over Crimea.
Russia has raised further concern by massing tens of thousands of troops at the border, but in an apparent bid to reassure his counterparts in Geneva, Lavrov said Russia had "no desire" to send troops into Ukraine.
"Ukraine has shown admirable, sometimes I think even remarkable restraint, in the face of considerable challenge," US Secretary of State John Kerry told reporters after hammering out the deal in Geneva with Lavrov, Ukraine's Andriy Deshchytsya and EU foreign policy chief Catherine Ashton.
While saying that the West is "not giving up" on Crimea, Kerry added that "we did not come (to Geneva) to talk about" the peninsula.
Instead, the four-parties' agreement included a call for an amnesty for those who vacated occupied buildings and surrendered weapons.
It added that a dialogue that included all Ukrainians regions should be launched -- apparently addressing Moscow's concerns that the rights of Russian speakers be assured.
"I can see why they (Russians) did this as they felt that the sanctions were quite close to being imposed, so they had to take a step back," Kiev-based political scientist Andreas Umland said.
"But I'm sceptical... I don't think it's all over."
No timeline was given for the implementation of the agreement, but Ukraine's foreign minister said it should begin "in the coming days".
Meanwhile, Kerry described as "grotesque" pamphlets which were sent to Jews in east Ukraine's main city, ordering them to register.
"In the year 2014, after all of the miles travelled and all of the journey of history, this is not just intolerable, it's grotesque. It is beyond unacceptable," he said.
The incident has been dismissed by the local chief rabbi as nothing more than "provocation".
Separately, the European Union announced Thursday it had agreed to hold talks with Russia on its gas supplies to Europe through Ukraine, warning Moscow its reliability as an energy source was at stake.
The announcement came even as Putin ramped up pressure on Ukraine by setting a one-month deadline for Kiev to settle its debt for gas imports from Russia.