(Adds comment from German government spokesman)
BERLIN May 7 (Reuters) - Ukrainian presidential candidate Petro Poroshenko urged Europe and the United States to agree on a third wave of sanctions against Russia if the Kremlin supports a referendum planned by separatists in eastern Ukraine on May 11.
Separatists in the Donbass region say the Sunday referendum on secession will be similar to the one that preceded Russia's annexation of Crimea in March. Such a move could complicate Ukraine's May 25 presidential elections.
"If Russia will support this referendum we need absolutely agreed action about the third wave of sanctions, well-coordinated between the United States of America and the European Union," Poroshenko told reporters in Berlin shortly before meeting Chancellor Angela Merkel.
A poll last month put Poroshenko, who supported the pro-European uprising which ousted Kremlin-backed president Viktor Yanukovich, on 48.4 percent, just short of the absolute majority needed to avoid a runoff in a second round.
The confectionery tycoon said his priority was to stabilise the situation in Ukraine, where fears of war are mounting, and left open the possibility of a referendum later.
"We are ready to have even a referendum but not under (the threat of) machineguns or automatic rifles," he said, adding people were afraid to visit polling stations.
"After the presidential election, after restoring law and order, we are ready to discuss any referendum and we are not afraid of a referendum, including the constitutional changes which I ... am ready to discuss with anybody."
Poroshenko also said the Russians "want to invite some separatists to the table" of a second round of Geneva talks but that these people had little support and the only legitimate representative of Ukrainians should be the foreign minister.
He said his visit to Merkel was not about her supporting his campaign to become president, but rather about the chancellor of Germany backing Ukraine's attempts to defend its sovereignty.
German government spokesman Steffen Seibert said earlier this week a Sunday referendum would violate Ukraine's constitution and worsen an already deadly situation.
"The chancellor expressed to Mr Poroshenko, as she does to all dialogue partners in connection with Ukraine, the importance of being willing to talk and capable of having a dialogue, especially in view of the election on May 25," Seibert said after Merkel's talks with Poroshenko.
"The OSCE should play a strong role in that."
The European Union and United States have warned Russia it will face additional sanctions against key sectors of its economy if Moscow disrupts Ukraine's plan to hold elections.
The West has already imposed several rounds of sanctions on specific Russians, including some members of Putin's inner circle, and several companies. (Additional reporting by Andreas Rinke and Stephen Brown; editing by Andrew Roche)