President Obama on Wednesday defended his administration’s decision not to authorize military intervention in Syria, despite the “natural instinct” to take action to halt the brutal crackdown by President Bashar al-Assad’s government.

After a private meeting with British Prime Minister David Cameron at the White House, Obama reiterated his demand for Assad to relinquish power immediately. But the president cautioned that the United States must think through “all of our actions” before taking more aggressive steps to intervene.

“When we see what’s happening on television, you know, our natural instinct is to act,” Obama said at a joint news conference with Cameron in the Rose Garden. The session was dominated by questions about Syria, Afghanistan and Iran.

But, Obama said, the “best thing we can do right now is to make sure that the international community continues to unify around the fact that what the Syrian regime is doing is unacceptable.”

The meeting between Obama and Cameron was dominated by foreign policy issues. The two leaders reaffirmed the bond between their nations, pledging to carry on with the NATO mission in Afghanistan as the United States works toward a gradual withdrawal of troops and the conclusion of combat operations by the end of 2014.

And they vowed to keep up international sanctions to try to force Iran to abandon its nuclear weapons ambitions, with Obama sternly warning that Tehran must meet its “international obligations or face the consequences.”

He added that “the window for solving this issue diplomatically is shrinking.”

In the case of Syria, the United States and its allies have sought to apply political pressure on Assad and provide humanitarian aid.

Cameron announced Wednesday that Britain will supply an additional $3.1 million in food and medical care, and he pledged to hold the Assad government responsible for war crimes after a transition of power is achieved.

“What is being done in Homs . . . is simply appalling and shouldn’t be allowed to stand in our world,” Cameron said of the Syrian city where the opposition was recently overrun. He said that British monitors in Turkey are cataloguing “these crimes” and warned that Assad should “always remember that international law has got a long reach and a long memory.”

The remarks came amid reports that the Syrian government had launched a renewed assault on opposition forces in Daraa, where the popular uprising against the Assad regime took root last year.

The Associated Press quoted a Jordan-based spokesman for the rebel Free Syrian Army, Muneef al-Zaeem, as saying that there were reports of dead and wounded, but that the numbers could not be confirmed.

Asked about Russia’s support for the Syrian government, including providing weapons to the regime, Cameron suggested that ordinary Syrians have begun to realize that Western powers, not Russia, are their true allies.

It is not in Russia’s interest, Cameron said, “to have this bloodied, broken, brutal regime butchering people nightly on the television screens.”

Obama added that the West must be careful not to cause “even more deaths” by intervening in a way that leads to civil war.

“The people who are going to ultimately be most affected by those decisions are the people in Syria itself,” Obama said.