By Natalia Zinets and Maria Tsvetkova
KIEV/DONETSK, Ukraine (Reuters) - The whole of Ukraine will be within reach of the "enemy" if its armed forces lose control of the international airport in the eastern city of Donetsk to separatists, President Petro Poroshenko said on Friday.
Poroshenko issued the warning after another night of intense fighting with Russian-backed rebels at the airport, part of a new spasm of violence in a conflict which has triggered the worst standoff between Moscow and the West since the Cold War.
Ukrainian troops made a tactical withdrawal in the face of rebel mortar and artillery strikes then returned to their positions, the Kiev military said.
Six Ukrainian soldiers were killed in clashes with the separatists in the past 24 hours, a military spokesman said, despite a three-month ceasefire.
The latest fighting, which included fresh shelling on a district of rebel-controlled Donetsk in which a pensioner was killed, came just days before both sides were due to try to reinforce the shaky ceasefire by observing a "Day of Silence" on Dec. 9.
Kiev and its Western backers accuse Russia of sending troops across the border and arming the rebels, charges Moscow denies.
Poroshenko made his comments at a Kiev ceremony in which he awarded medals to some of the defenders who have hung doggedly on to control of the airport since May.
"I am sure that we are defending there the whole of Ukraine," he said. "If we give up Donetsk (airport), the enemy will be at Borispil or Gostomel or even in Lviv," he said.
Borispil is the name of the capital's main international airport and Gostomel is a small former military airfield outside Kiev. Lviv is Ukraine's main city in the west.
Military spokesman Andriy Lysenko said the separatists had suffered "big losses" in three failed attempts to break through Ukrainian positions, including at Donetsk airport.
Ukrainian and separatist leaders have pledged to observe the "Day of Silence", intended to prepare for creation of a non-militarized buffer zone, from which the two sides will withdraw artillery and other heavy military equipment.
The United Nations puts the overall death toll in the conflict at more than 4,300.
Russian Foreign Minister Sergei Lavrov said on Friday agreeing a demarcation line between the rival sides was the crucial element that had stalled earlier truce efforts.
"We expect the ceasefire to take effect in full from Dec. 9 ... The ceasefire was announced before. It led to a considerable easing in violence but not to a full (halt of fighting)," Lavrov said during a visit to Basel, Switzerland.
Ukrainian Foreign Minister Pavlo Klimkin, also speaking in Basel, said: "We have human losses and human suffering every day. So it's about real delivery, it's about deeds and not just commitments and words."
(Additional reporting by Gabriela Baczynska in Moscow and Joshua Franklin in Basel; Writing by Richard Balmforth)