"We have 100 percent information that Assad regime used chemical weapons against rebels", said Israeli Defense Minister Avigdor Lieberman, appearing at the press conference with Mattis.
The US Defense Secretary James Mattis said during a visit to Tel Aviv that in recent days the Syrian air force "has dispersed its combat aircraft".
There also is evidence that the Islamic State of Iraq and the Levant (ISIL) group and other armed anti-government groups have acquired chemical weapons.
On April 7, the US launched dozens of cruise missiles at Shayrat air base in Syria in response to a chemical weapon attack in Idlib province three days earlier that killed almost 100 people, including 30 children.
Russian Federation has told the United States it regrets Washington's opposition to letting its inspectors take part in an investigation into a chemical weapons attack in Syria earlier this month, the foreign ministry said on Friday.
The measure follows an initial round of sanctions announced last week, Canada's first against Syria and its president, Bashar al-Assad, since 2014, when a conservative government was in office in Ottawa.
Netanyahu also noted that Mattis had "very clear and forthright words" about Iran.
Asked about the possibility of expanding Russia's role in Syria, Assad said in an interview with the RIA Novosti news agency on Friday that "what has been done so far is good and sufficient". Top Assad ally, Russia, has asserted a Syrian government airstrike hit a rebel chemical weapons factory, causing the disaster.
Adding the new names to the sanctions list is part of additional global pressure on the Assad regime to immediately stop the "repeated and heinous attacks against its own people", the government said.
"It is a really great honor, a pleasure, to welcome you secretary of defense, not only as the secretary [of defense] of our greatest ally, but as a former general, and not only as a general, but as a real soldier, and a leader", he said. Chemical weapons have been taken out of the country with US' help.
Mattis later met Israeli Prime Minister Benjamin Netanyahu in Jerusalem.
Some U.S. officials believe nearly all of Syria's military aircraft have been relocated into close proximity with the Russian base to make it harder for U.S. missile strikes to wipe out the Syrian air force. And of course, Iran's got its own military inside Syria, continuing to hold Assad in power. On Tuesday, the US State Department certified that Iran was in full compliance with the agreement, but indicated that the administration was conducting full review of its sanctions regime to determine whether its waiver of sanctions under the terms of the deal remained "vital to the national security interests of the United States".