Moscow sees the deployment of an additional 250 US military personnel in Syria as an affront to Damascus' territorial sovereignty, Russian Deputy Foreign Minister Sergei Ryabkov told Russia's state-owned TASS news agency on Friday.

"If we are talking from the position of the foreign minister, we - of course - cannot not be concerned over the fact that the US carries out such actions without coordination with the legitimate government of the Syrian Arab Republic," said Ryabkov.

"I stress that officially Damascus already gave a similar political assessment, and we fully agree with it," the minister added.

US President Barack Obama on Sunday announced that he had decided to send additional US special operations troops to support the 50 already on the ground. Obama said the unit will train local forces to fight the self-styled "Islamic State" militant group.

However, Damascus on Thursday rejected the move as a "direct attack" on Syria, saying it was deeply concerned about news reports suggesting 150 US troops entered the country this past week.

"This intervention is rejected and illegitimate, and it happened without the Syrian government's approval," an unnamed foreign ministry official told the official Syrian Arab News Agency.

Violence in Aleppo

Meanwhile, the UK-based Syrian Observatory for Human Rights late Thursday reported that nearly 200 civilians have been killed since April 22, when violence escalated between rebel and government forces in and around the northern city of Aleppo.

The Observatory's report came shortly after warplanes bombed a hospital in Aleppo, leaving at least 30 dead.

Russia and the Syrian government denied responsibility for the incident, while US State Secretary John Kerry called on Moscow to pressure Damascus into ceasing hostilities.

The US and Russia brokered a "cessation of hostilities" that went into effect on February 27. However, the ceasefire has witnessed increased violations since mid-April.

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'Duty to protect civilians'

UN human rights chief Zeid Raad al-Hussein on Friday slammed the regime and rebel forces for the increase in civilian casualties.

"The violence is soaring back to the levels we saw prior to the cessation of hostilities. There are deeply disturbing reports of military build-ups indicating preparations for a lethal escalation," al-Hussein said in a statement.

"These indicate a serious, alarming disregard for one of the cornerstones of international humanitarian law: the duty to protect civilians," he added.

More than 270,000 people have been killed and half the population displaced since 2015, when the conflicted erupted following a violent government crackdown on pro-democracy protesters calling for President Bashar al-Assad to step down.