A suicide bombing that killed five policemen in Chechnya is a stark reminder of the Islamist insurgency spreading in the North Caucasus and the threat posed by jihadists who have joined the Islamic State group, analysts say.

"The number of shootings and deaths has dropped but the situation remains far from stable," said Grigory Shvedov, editor of the website kavkaz-uzel.ru that focuses on the Caucasus.

The first unarmed drones arrived Monday in Ukraine to help OSCE teams monitor the implementation of a fragile one-month truce in the separatist east, the pan-European security group said.

Ukrainian forces and pro-Russian separatist fighters marked one month on Sunday since the signing of a Kremlin-backed truce with one the most heated battles of the six-month war in the ex-Soviet state's eastern rustbelt.

Artillery blasts rocked Donetsk on Sunday morning, exactly one month since the rebels signed a 12-point agreement with Kiev's representatives to try to halt the hostilities that have killed over 3,300 people in eastern Ukraine.

A convoy of more than 100 trucks carrying humanitarian aid has left Germany for war-torn eastern Ukraine, the German ministry of economic cooperation and development said Saturday.

The convoy should arrive in mid-October and on October 14 Cooperation Minister Gerd Mueller is also expected to head to the city of Kharkiv, which remains under government control.

Russia came under mounting pressure from the West on Saturday to save Ukraine's crumbling truce with pro-Kremlin insurgents who are engaged in intensifying raids on an airport vital for sustaining their independence drive.

But analysts said Russian President Vladimir Putin was ready to weather isolation and economic sanctions as the cost of cementing his grip on Ukraine's industrial east. 

Latvia's ruling center-right coalition led by Prime Minister Laimdota Straujuma scored a resounding majority in Saturday elections overshadowed by alarm over a resurgent Moscow and a Kremlin-allied party popular with the country's sizable Russian minority.

An exit poll by the SKDS agency gave three parties in her coalition 63 seats in the 100-member parliament, while their Kremlin-allied leftist rival took 23.

Russia will modernize and relaunch a Soviet-era radar station on the Crimean peninsula annexed from Ukraine to provide early warning of missile strikes, a senior defense official said Saturday.

The radar station in the port city of Sevastopol will become fully operational in 2016, the commander of the air and space defense forces, Alexander Golovko, was quoted as saying by the TASS news agency.

Ukrainian President Petro Poroshenko on Friday risked further angering the Kremlin by suggesting that English lessons replace Russian ones in schools to improve the country's standard of living.

"English should become the second language to be taught in schools," Poroshenko said on a visit to Lviv, a nationalist bastion of political support for the new pro-Western leader where Russian speakers make up a minority.

Bulgarian President Rosen Plevneliev denounced Russia's Vladimir Putin as an aggressive leader with a 19th-century empire-building style, in comments to be published Saturday.

"Russia is a nationalistic and aggressive state," Plevneliev told Germany's Frankfurter Allgemeine Zeitung.

Shelling in the center of the main rebel-held city in Ukraine's east killed a Swiss Red Cross worker Thursday, tearing badly at a weakly observed four-week truce meant to defuse Europe's worst crisis in decades.

The attacks in Donetsk were the first to strike the heart of the city since the signing of the truce on September 5 between Kiev and Moscow.