The OSCE's Special Monitoring Mission (SMM) to Ukraine says it has seen a "massive" increase in the number of cease-fire violations in eastern Ukraine in recent days.
Michael Bociurkiw, the OSCE mission's spokesman, said on April 15 in an interview with the BBC from Kyiv that monitors also reported "intensive fighting" between Russian-backed separatists and Ukrainian forces near the airport outside the rebel-held provincial capital of Donetsk and around the village of Shyrokyne, near the strategic government-held port city of Mariupol.
Bociurkiw said the SMM monitors recorded more than 700 explosions near the Donetsk airport during a six-hour period on April 14.
He added that the fighting sides are within a few hundred meters of each other near Shyrokyne and that shelling is coming from "densely populated residential areas."
Shyrokyne is just 10 kilometers from Mariupol, and the rebel attacks there have raised concerns that the separatists may seek to take the largest government-held city in the Donetsk region and push further westward toward Crimea, the Black Sea peninsula that Russia illegally annexed from Ukraine in March 2014.
Bociurkiw said Ambassador Ertugrul Apakan, the head of the OSCE's SMM to Ukraine, has reiterated a call for a "comprehensive cease-fire" that includes the withdrawal of heavy weaponry.
"We've seen yet again," he said, "movement of heavy weaponry that should have been completely removed from the zone in which it's being used."
The SMM in Ukraine officially monitors the fragile cease-fire agreed to in February in Minsk that has reduced the fighting in eastern Ukraine, which has killed more than 6,000 people in the past year.
In Kyiv, Ukrainian Prime Minister Arseniy Yatsenyuk said at a government meeting on April 15 in Kyiv that he was "deeply frustrated" that Russia is "failing to implement the Minsk agreements."
He said "it looks like only Ukraine" is adhering to the cease-fire agreement, adding that Ukrainian forces are suffering losses after a period of no casualties and "we have the shelling again."
Yatsenyuk told cabinet members that Kyiv was willing to implement all of the aspects of the Minsk peace agreement, including the holding of "fair and transparent" elections in the rebel-held areas of the Donbas region.
He said Ukraine will use all means to "struggle for peace," including by improving the military's combat capability.
Meanwhile, representatives of the separatists criticized Ukraine for refusing to have direct dialogue with the self-declared leaders of the rebel groups that control parts of Ukraine's Luhansk and Donetsk regions.
Ukrainian Foreign Minister Pavlo Klimkin said in a television interview late on April 14 that Kyiv would only initiate such a dialogue after free and fair elections under Ukrainian law are held in the separatist-held areas.
He added that the current leaders are illegitimate because neither Ukraine nor the international community recognize the November 2 elections held by the separatists.
But Denis Pushilin, a Donetsk separatist leader, said Kyiv's refusal to talk to rebel representatives contradicted the Minsk agreement.
Meanwhile, Ukrainian National Security and Defense Council spokesman Andriy Lysenko said Russia has concentrated troops near the Ukrainian border in an area north of the rebel-held city of Luhansk, at Veydelevka, near the Ukrainian city of Kharkiv.
He said there are some 50 units of armored equipment and around 2,000 troops just 20 kilometers from the Ukrainian border.
In Germany, top diplomats of the seven leading industrial countries have gathered in the northern port city of Luebeck for two days of talks.
Host Frank-Walter Steinmeier said on April 14 that the G7 foreign ministers would look to build on recent progress on the Ukraine crisis and Iran's nuclear program.
The G7 includes Britain, Canada, France, Germany, Italy, Japan, and the United States.
Formerly the G8, the grouping was reduced to seven members after Russia's annexation of Crimea in March 2014.
Ahead of the G7 meeting, Steinmeier pinned Russia's chances of returning to the group on an end to the conflict in Ukraine.
Moscow must help to ensure the "Ukraine conflict moves closer to a solution," he said.
Steinmeier added that Russia needed "very urgently" to set a new course to deal with other conflicts such as in Syria and Iran.
With reporting by the BBC, dpa, and Reuters