By Pavel Polityuk and Aleksandar Vasovic
KIEV/NOVOHANNIVKA Ukraine (Reuters) - Ukraine's president promised a tough response on Saturday to pro-Russian separatists who shot down an army transport plane, killing 49 servicemen and dealing a blow to a military campaign to crush their uprising.
Newly installed Petro Poroshenko summoned security chiefs for consultations after the plane was hit by an anti-aircraft missile as it came in to land at an airport outside the city of Luhansk, a center of the rebellion in east Ukraine.
"All those involved in cynical acts of terrorism of this magnitude must be punished," he said, declaring Sunday a day of mourning for the nine crew and 40 paratroopers killed.
He later issued a separate statement saying he had called another meeting of his security chiefs for Monday, and that the armed forces had already intensified their operation - intended to prevent Ukraine breaking up.
"For the sake of peace, we will act decisively and purposefully," he said, hailing the seizure of the port city of Mariupol from the rebels on Friday and the recapture of 248 km (155 miles) of the frontier with Russia "across which the terrorists get weapons, equipment, reinforcements and money."
Charred debris was scattered for hundreds of meters (yards) over the sloping wheat field where the plane came down near Novohannivka, a village 20 km (12 miles) southeast of Luhansk.
The tail section jutted up from the ground, with pieces of the engines, fuselage and other parts lying around it. A platoon of rebel forces in camouflage scoured the ruins for ammunition.
"This is how we work. The fascists can bring as many reinforcements as they want, but we will do this every time. We will talk to them on our own terms," said a stocky 50-year-old rebel who identified himself as Pyotr, his 'nom de guerre'.
He had an assault rifle in one hand, a light machine gun in the other and two ammunition belts round his neck.
RUSSIAN TENSIONS WITH THE WEST
The death toll was the highest suffered by government forces in a single incident since the crisis flared in February and is likely to fuel tension between Russia and Kiev's main ally, the United States, which accuses Moscow of arming the rebels.
In a telephone call with Ukrainian Prime Minister Arseny Yatseniuk, U.S. Secretary of State John Kerry expressed condolences for the servicemen's deaths, a senior State Department official said.
Kerry also spoke to Russian Foreign Minister Sergei Lavrov and warned him that the United States and its G7 partners would "raise the costs" Moscow could face unless it curbed weapons supplies into Ukraine and cut ties with the separatists.
The United States and the European Union have imposed sanctions on Russia over Ukraine - largely limiting them to visa bans and asset freezes on some individuals, banks and companies - but said they could get tougher if Moscow does not respond.
"We condemn the shooting down of the Ukrainian military plane and continue to be deeply concerned about the situation in eastern Ukraine, including by the fact that militant and separatist groups have received heavy weapons from Russia, including tanks, which is a significant escalation," a White House spokeswoman said.
In Kiev, protesters pelted the Russian embassy with eggs and ripped up a Russian flag in protest of what they called Moscow's backing of the separatist rebels in east Ukraine. Washington condemned the attack.
Although Russian President Vladimir Putin denies Moscow is behind the uprising, German Chancellor Angela Merkel and French President Francois Hollande expressed dismay over the attack in a three-way telephone call with him.
Echoing the U.S. criticism, the German government said Russia must control its borders effectively to prevent arms and combatants entering Ukraine and use its influence with the rebels to secure and protect a truce.
The separatists say they get their weapons from looted military stockpiles. They shot down a military cargo plane last week, killing three people, and a general was among 14 killed when an Mi-8 transport helicopter was hit on May 29.
Despite the continuing violence, Ukraine and Russia have begun talks on a peace plan and Moscow made a goodwill gesture by agreeing to make a last attempt to solve a gas pricing dispute before a Monday deadline to cut off supplies to Kiev.
Talks broke up shortly after they began in Kiev on Saturday evening but were due to resume on Sunday. Cutting off supplies to Kiev could also cause disruptions to deliveries to the European Union, which gets half its gas imports from Russia through Ukraine.
The talks have been complicated by tension over the uprising in Russian-speaking east Ukraine, which the rebels want Moscow to annexe, as it did Crimea in March. Their calls for a Russian invasion have gone unanswered.
Russia fears losing influence in Ukraine following the overthrow of its Moscow-leaning president in February and its new leaders' pro-Western policies. Ukraine was ruled from Moscow in Soviet times and is seen by Russians as the cradle of their civilization.
(Additional reporting by Alessandra Prentice in Donestsk, Guy Trompiz in Paris, Annika Breidthardt in Berlin and Jeff Mason; Writing by Timothy Heritage; Editing by Eric Walsh)