In Washington, meanwhile, President Obama is close to deciding to give military aid to Syrian rebels and likely will make a final decision within weeks, senior officials said.
Rebel activists said Tuesday's attack targeted buildings belonging to Ahrar al-Sham, a Salafist Islamist rebel group fighting to oust Syrian President Bashar Assad, Hurriyet Daily News reported Wednesday.
A Turkish aid worker said a warehouse used by aid groups on the Syrian side of the border also was hit.
"The target appears to be Ahrar al-Sham but most of the fighting brigades have a presence at and around the crossing and it is impossible to get them without harming civilians," said an activist identified as Mohammad.
Another Syrian activist at Bab al-Hawa said people waiting to cross into Turkey were among those hit. He said the dead included an 18-month-old baby and two teenagers.
In Washington, one White House official told The Washington Post, "We're clearly on an upward trajectory. We've moved over to assistance that has a direct military purpose."
The officials did not spell out what military-purpose U.S. assistance was under consideration, but rebels have specifically requested anti-tank weapons and surface-to-air missiles.
Supplying arms was one of several options under consideration, the officials stressed to the Post, adding diplomacy remained the preferred option.
Obama spoke of possibly escalating assistance to the rebels during a news conference Tuesday.
"There are options that are available to me that are on the shelf right now that we have not deployed," he said. "And that's a spectrum of options.
"As early as last year, I asked the Pentagon, our military, our intelligence officials, to prepare for me what options might be available," Obama said. "And I won't go into the details of what those options might be, but clearly that would be an escalation."
Confirmation of the Assad regime's use of chemical weapons would mean "there are some options that we might not otherwise exercise that we would strongly consider," he said.
Obama emphasized the need to "make sure I've got the facts. That's what the American people would expect. ..."
"If we end up rushing to judgment without hard, effective evidence, then we can find ourselves in a position where we can't mobilize the international community to support what we do," he said.
To that end, the administration has begun an effort to convince Russian President Vladimir Putin to reconsider his support of Assad in light of the regime's probable use of chemical weapons, the Post said.