Warning: This story contains images some readers may find distressing.
Bulgaria's Foreign Ministry said that it was following with concern information about another chemical weapon attack in Syria. They deny that the Syrian government was behind the attacks as it fully destroyed its chemical weapons in 2013 under global supervision.
Russian Federation has been a key ally of Bashar Assad and has been waging an air campaign in support of his forces since 2015.
"The Assad regime and its backers must be held accountable and any further attacks prevented immediately", she added.
Trump threatened action, although it was unclear what he had in mind.
Saturday's suspected poison gas attack on the besieged town of Douma came nearly exactly a year after the US missile attack prompted by the Khan Sheikhoun deaths.
He said: "President Putin, Russia and Iran are responsible for backing Animal Assad".
Earlier, President Donald Trump had promised a "big price to pay" for the suspected chemical attack.
"We tried to send people to the area to rescue the injured, but even the rescue workers began suffocating", said Mohamed Samer, a medical worker in Douma.
Russian Foreign Minister Sergei Lavrov said Monday that no evidence was found of a chemical attack. "His calculation is based on whether it will help his chances in achieving gains on the ground, or punishing the rebels", he said.
United Nations war crimes investigators had previously documented 33 chemical attacks in Syria, attributing 27 to the Assad government, which has repeatedly denied using the weapons.
Last week, Turkish President Recep Tayyip Erdogan welcomed his Iranian and Russian counterparts in Ankara for a summit on Syria.
The US has about 2,000 troops in Syria, where they advise local forces fighting ISIS. "The question now is whether he will do anything about it", McCain said in a statement.
White House security adviser Tom Bossert refused to rule out United States military action in response to the alleged chemical strike.
Footage posted by the group online, which was not possible to verify, showed victims including children foaming at the mouth. "Douma city, April 7. there is a strong smell here", a voice can be heard saying.
The reports have not been independently confirmed. Six others had died while receiving treatment.
"The children of Syria have witnessed and experienced unimaginable suffering over the past seven years". It said the death toll was likely to rise.
Syrian activists, rescuers and medics said a poison gas attack killed at least 40 people, with families found suffocated in their houses and shelters. "Meeting expected on Monday".
Bolton past year praised Trump's missile response, though he has generally focused more on Iran as a bigger security threat.
Indeed for neoconservative hawks such as Trump's newly appointed National Security Advisor, John Bolton, the world is America and America is the world, with those who dare resist ripe for attack. Christopher Sherwood, Pentagon spokesman stated that this time the Department of Defense is not conducting airstrikes in Syria.
In an appearance on ABC's "This Week", Bossert said U.S. officials have examined photos and information about the incident in Douma, a Damascus suburb.
- OPCW executive council adopts decision condemning Syrian government and Islamic State for chemical weapons use after a vote that split the body and signalled an end to US-Russia cooperation.
August 2013:Rockets containing nerve agent Sarin were fired at Eastern Ghouta.
- The Syrian government of President Bashar al-Assad is blamed by the Joint Investigative Mechanism for the sarin attack on opposition-held Khan Sheikhoun that killed dozens of people.
It said the attack left people dead and wounded.
"Children are dying before our eyes", said UNICEF Executive Director Henrietta Fore.
In the past few weeks, the government troops regained control of large pockets of territory in Eastern Ghouta, held by the anti-government armed groups for almost over five years.
The attack took place in a town called Douma, which is in Syria's region of Eastern Ghouta.