Ukraine's military scrambled aircraft and paratroops to confront Russian troops landing on a spit of land between Crimea and the mainland, defence officials said and the foreign ministry demanded their immediate withdrawal.
The border guard service said Ukrainian forces had taken up defensive positions on Arbatskaya Strelka, running parallel to the east of Crimea, now controlled by Russian forces. It said about 60 Russian troops had landed on the strip and begun digging in, assisted by three armoured personnel carriers.
Six Russian helicopters arrived with 60 more servicemen about two hours later outside the village of Strelkovoye. The border guard service said talks between the two sides established that the Russian servicemen were "guarding against possible terrorist acts" against a gas pumping station.
"At this time, there is no threat of confrontation," the Ukrainian border guard service said.
The defense ministry had earlier said troops dispatched to Arbatskaya Strelka had "immediately" repelled the incursion.
Ukraine's foreign ministry said the village had been occupied by Russian forces, though the border guards gave a different version. "The ministry is making public the Russian incursion and demands the immediate withdrawal of Russian armed forces from Ukraine's territory," it said.
Crimea is home to Russia's Black Sea Fleet under a post-Soviet deal signed with Ukraine. The peninsula has been under the control of Russian forces for two weeks.
Two main roads link Crimea to the mainland, at Armyansk in the west and Chonhar in the east. However, there is a third, minor road connection from the 120 km (75 mile) long Arbatskaya Strelka. Strelkovoye is 30 km (20 miles) south of the town of Henichesk on the Ukrainian mainland.
Earlier, Ukraine's acting president accused "Kremlin agents" of fomenting deadly violence in eastern cities, in one of the most direct and highest-level criticisms aimed by Kiev at President Vladimir Putin.
From his speaker's chair in parliament, interim head of state Oleksander Turchinov referred to three deaths in two incidents this week in Donetsk and Kharkiv and told opposition lawmakers: "You know as well as we do who is organizing mass protests in eastern Ukraine - it is Kremlin agents who are organizing and funding them, who are causing people to be murdered."
Turchinov has warned of a risk of a Russian invasion of the east following Moscow's occupation of the Crimea peninsula. Ukrainian officials have called on people in the mainly Russian-speaking cities of the industrial east not to rise to provocation that Russia might use to justify sending in troops.
For his part, Ukraine's acting interior minister said two people have been killed and several wounded in a shootout in the eastern Ukrainian city of Kharkiv.
Arsen Avakhov wrote on his official Facebook page early Saturday that around 30 people "from both sides" were arrested during a shootout late on Friday.
Russian state news agency Itar Tass reported that the clash occurred outside a building of the far-right Ukrainian nationalist group, Right Sector, although Avakhov made no mention of the group and said the incident was under investigation.
Violence has escalated in Ukraine's Russia-leaning east in recent days, as pro-Russia demonstrators have seized government buildings and clashed with supporters of the new Kiev government. At least one person died and 17 were wounded in clashes in the city of Donetsk on Thursday.
UN resolution
Russia vetoed a UN Security Council draft resolution that declares a planned referendum on the status of Ukraine's Crimea region "can have no validity" and urges nations and international organizations not to recognize it.
The Russian veto of the draft resolution, drawn up by the United States, was expected. Moscow, which has sent military forces to the Crimea, is backing Sunday's referendum, which would transfer control of the region from Ukraine to Russia.
The brief resolution noted that the referendum was not backed by the Ukrainian government in Kiev.
"This referendum can have no validity, and cannot form the basis for any alteration of the status of Crimea," it states. It calls on "all states, international organizations and specialized agencies not to recognize any alteration of the status of Crimea on the basis of this referendum."
Russia's ambassador to the United Nations, Vitaly Churkin, told reporters before the council met, "No surprises at this vote."
Russia is one of the Security Council's five permanent, veto-holding members.
France
French President Francois Hollande said on Saturday Paris would review its military cooperation with Russia as part of a third level of sanctions if Moscow did not de-escalate the crisis in Ukraine.
On Monday the United States and European Union are expected to unveil a list of Russian officials subject to asset freezes and visa bans as Western nations attempt to step up pressure on Moscow over its intervention in the Ukrainian region of Crimea.
When asked whether France would suspend a 1.2 billion euro helicopter carrier contract with Russia, Hollande told a news conference: "As far as other sanctions, notably military cooperation, that is the third level of sanctions."
US
The USS Truxtun, a U.S. guided-missile destroyer, will carry out more exercises with allied ships in the Black Sea, its commander said on Saturday, in a further sign of the international response to Russia's actions in Ukraine.
Commander Andrew Biehn was briefing reporters aboard the 300-crew destroyer as it lay docked in a Bulgarian port.
The USS Truxtun last week took part in drills with Romanian and Bulgarian ships a few hundred miles from Russian forces that entered Ukraine's Russian-majority of Crimea after mass protests toppled the country's pro-Moscow president.
REUTERS/AP