* Incident took place at separatist-manned checkpoint
* Shakes hope of Easter truce
* Takes place as OSCE probes disarming prospects (Adds varying casualty figures, colour, changes dateline)
By Aleksandar Vasovic
SLAVIANSK, Ukraine, April 20 (Reuters) - Gunfire erupted near a makeshift checkpoint manned by pro-Russian separatists near Slaviansk in eastern Ukraine early on Sunday, killing at least two people.
Separatists gave a higher casualty figure, saying they had come under attack from Ukrainian nationalist paramilitaries.
The incident represented the first fatalities in eastern Ukraine since a four-way agreement was reached last Friday in Geneva to de-escalate the situation in eastern Ukraine where separatists are rebelling against Kiev's pro-western leaders.
Though police on the interior ministry website said one person was killed and three others received gunshot wounds in the incident involving "two groups of civilians", Reuters witnesses at the scene saw two dead bodies in a truck.
A separatist fighter put the death tally higher, saying the attackers - whom he said were Ukrainian ultra-nationalists - killed one separatist immediately and two others in ensuing shooting after arriving in cars at the checkpoint and opening fire.
He said two of the attackers, whom he said were from the far-right nationalist Right Sector group which figured prominently in the overthrow in Kiev of Moscow-backed president Viktor Yanukovich in February, were killed after the separatists brought in extra men.
Two Reuters journalists at the scene saw two dead bodies, one of them with gunshot wounds in the head and face. One man wore combat fatigues and the other was in civilian clothes. Their bodies were lying in a truck, one of them covered with a blanket.
Local people were laying flowers at the scene and touching the bodies in a gesture of farewell.
The reported incident will shake hopes of an Easter truce and a speedy demilitarisation of the restive region under the Geneva accord worked out between the United States, Russia, Ukraine and the European Union.
Monitors from the OSCE human rights watchdog have moved into the area to try to persuade pro-Russian separatists to lay down their arms.
Armed separatists are in control of Slaviansk, one of 10 or so points in the Russian-speaking east of Ukraine where localised rebellions have broken out against Kiev's pro-Western leaders following Yanukovich's overthrow.
Interior Minister Arsen Avakov had no word of the reported incident on his Facebook page, where he usually posts updates on any clashes.
A Reuters photographer at the scene said there were two burned out jeeps at the barricaded makeshift checkpoint which was near a group of houses about 18 km (11 miles) west of the city.
The rebels, in action which Kiev's new rulers say is inspired and organised by Moscow, have occupied state buildings including police headquarters and state security offices in several localities in eastern Ukraine and declared support for an independent "Donetsk people's republic".
The separatist fighter, who gave his name as Vladimir, said four cars approached the checkpoint at around 2 a.m.
"One of our men approached them. They shot him in the head and he fell immediately. There was some sniper fire. We had 3 dead and four wounded."
He said the separatists brought in reinforcements and in subsequent shooting two of the attackers were killed.
(Additional reporting by Dmitry Madorsky and Gleb Garanich in Slaviansk; writing by Richard Balmforth; editing by Philippa Fletcher)