KIEV, Ukraine (CNN) -

All the trucks sent over the border by Russia on a humanitarian mission without Ukraine's authorization have now returned to Russia, international monitors said Saturday.

In total, 227 vehicles were sent into eastern Ukraine on Friday, according to the Organization for Security and Co-operation in Europe, which has an observer mission at the checkpoint the convoy went through.

By Saturday afternoon, they had all returned to Russia after delivering aid to the city of Luhansk, a stronghold for the pro-Russia rebels which has been caught up in conflict.

Russia said the vehicles were on an essential humanitarian mission, but international powers condemned it as a violation of Ukraine's sovereignty.

A senior Ukrainian official characterized the convoy as a "direct invasion" of his nation by its mighty neighbor.

All the vehicles were supposed to be monitored by members of the International Committee of the Red Cross. However, the Red Cross said it wasn't accompanying them due to the "volatile security situation" -- a reference to continued fighting between pro-Russia rebels and Ukrainian forces.

Russia's Foreign Ministry said it was satisfied that its humanitarian convoy had delivered supplies to its destination. Moscow intended to "continue to cooperate with the ICRC" in aid efforts in eastern Ukraine, its statement said.

However, Ukraine's National Security and Defense Council gave a rather different assessment.

It said Saturday that preliminary information indicated that many of the empty trucks were filled up with military equipment from arms factories that are now in rebel hands. Those weapons were then carried back over the border.

Russia's defense industry has relied heavily since Soviet times on the arms factories in eastern Ukraine.

Ukraine has repeatedly accused Russia of directly and indirectly bolstering the rebel movement in the Donetsk and Luhansk regions, allegations Moscow denies.

In a statement Saturday, OSCE chairman Didier Burkhalter deplored the failure to deliver the aid according to the agreements made between Russia and Ukraine.

Appealing to all sides to cooperate in helping civilians impacted by the fighting in Luhansk and Donetsk, he urged them "to exercise maximum restraint and refrain from any actions which could contribute to the further escalation of the situation."

But violence erupted Saturday night when shelling in Donetsk left three people dead and three others injured, according to the Donetsk mayor's office. Journalists on the scene said one of the three was a child. Bystanders claimed all were members of one family.

Merkel: Can't rule out more sanctions

Amid the furor, German Chancellor Angela Merkel met in Kiev on Saturday with Ukrainian President Petro Poroshenko.

At a joint news conference, Merkel said she could not rule out further sanctions against Russia if no progress is made in resolving the situation in eastern Ukraine.

Germany and the European Union would continue pursue a diplomatic solution in talks with Russia and Ukraine over the crisis, she said.

But, she said, Ukraine's territorial integrity is essential and its border must be secured. "We need a peaceful situation but there's no use if there's an open border with Russia over which arms are coming," she said.

Poroshenko said the Russian convoy had "violated every international law," but that confrontation had been avoided thanks to Ukraine's efforts.

He accused foreign mercenaries of bringing war to Ukraine and said they did not have the support of the people of eastern Ukraine.

"Now the time has come for peace," he said. "Our government, together with our European partners, will be doing everything for that to happen, but not at the price of Ukrainian territorial integrity."

He said he was committed to constitutional reforms and decentralization of power aimed at meeting the concerns of the Russian-speaking population in eastern Ukraine.

Merkel said millions of dollars in international aid for the region would help rebuild infrastructure and homes damaged in the conflict.

The Ukrainian President is due to meet with Russian President Vladimir Putin and EU representatives on Tuesday in Minsk, Belarus, for talks.