By Esra Kaymak Avci

WASHINGTON

The U.S. is worried about an uptick in violence in Syria, according to a spokesman for the State Department.

Mark Toner said Monday that Washington was "very, very concerned about the recent increase in violence," including actions in contravention of a cessation of hostilities agreement.

He added that the "vast majority of violations" had been on the part of Bashar Assad's regime.

On Sunday, Sec. of State John Kerry expressed similar concerns to his Russian counterpart Sergey Lavrov, Toner said, and that "every extra effort is made" to sustain the truce.

"That includes working to define where the different fighting groups are located, and to make sure we're concentrating our efforts on Daesh and Nusra, who have not signed onto the cessation of hostilities," Toner said.

Toner's comments come after the Syrian army allegedly sent reinforcements to Aleppo, the largest city in Syria .

That could harm the success of peace talks set to resume April 13 in Geneva as Aleppo is the scene of fighting between opposition and regime forces.

"We want to make sure the environment for these talks to succeed in is a good one. And so, we are concerned," Toner said.

The truce was agreed to in February between the U.S., Russia and other members of the International Syria Support Group. The U.S.-led coalition backs the Syrian opposition but Russia supports the Assad regime.

The Geneva talks, under the auspices of the U.N., seeks to bring together the Syrian government and the opposition to find a road map for Syria's political transition.

Prior talks could not proceed as there was no agreement between sides as the opposition had preconditions such as a cease-fire and humanitarian aid delivery to besieged areas.

These were partly achieved, but moreover, sides can not agree whether there should be a transition with Assad.

Earlier Monday, the U.N. Special Envoy for Syria, Staffan de Mistura, said the upcoming talks will cover political transition, governance and the constitution.

Toner said the U.S. wanted to see opposition forces not attacked as the Syrian army seeking to take Aleppo.

"If they are attacking members of the Syrian opposition who have signed on to the cessation of hostilities, then those are violations of the cessation of hostility," he stated.

Toner added that the U.S. welcomed Russia's efforts to defeat Daesh but "needed greater clarity about what is actually planned, who are they targeting in [Aleppo]".

When asked about the upcoming parliamentarian elections in Syria on April 13, Toner described them as "premature," adding it could not be illegitimate as they don't represent the will of Syrian people.

He said the U.S. still believed Assad could not be part of any future Syria, but there has to be "some kind" of governance until a new one is formed.

Toner's comment came after Kerry's recent statement that the U.S. believed Assad has a role to play during the transition period.