The US has stepped up condemnation of the interim Egyptian government as its bloody crackdown on protesters put the Obama administration under growing pressure to rethink its tacit support for the new military rulers.

In its harshest criticism yet, the White House accused the military of breaking its promises to respect the rights of all citizens, and said it was "on the wrong path".

"Violence will only make it more difficult to move Egypt forward on a path to lasting stability and democracy, and runs directly counter to the pledges by the interim government to pursue reconciliation," spokesman Josh Earnest said.

"We also strongly oppose a return to a state of emergency law, and call on the government to respect basic human rights such as freedom of peaceful assembly, and due process under the law. The world is watching what is happening in Cairo."

But the White House once again avoided using the word "coup" to describe the recent overthrow of president Morsi – a move which would trigger an automatic congressional ban on US aid to the Egyptian military.

Later, secretary of state John Kerry appealed for calm. "Egyptians inside and outside the government need to take a step back and calm the situation and avoid further loss of life," he said.

Secretary of state John Kerry. Photograph: Evan Vucci/AP

"All parties share the responsibility for avoiding violence. There can only be a political solution. This is a pivotal moment for all Egyptians"

He said he had spoken to a number of other foreign ministers, including Egypt's. "I am confident that a path to peace is still open but it has been made much much harder by the events of today," Kerry said.

"The US remains at the ready to work with all of the parties," he added.

Kerry also spoke to Mohamed ElBaradei, the interim vice-president and former diplomat who resigned on Wednesday in protest at the crackdown. The state department described his resignation as "concerning".

Washington has suspended a recent shipment of F16 jets and said it was re-assessing whether to restart its $1.3bn of military aid, but its fears that a permanent severing of aid risks removing the only leverage it has in restraining the generals.

Nonetheless, Wednesday's bloody crackdown is straining hopes in Washington that the interim military government represents the best prosepct of a smooth return to democracy.

The White House was even asked by reporters whether it would back the return of Morsi to power.

"We are not in a position to throw our support behind specific politicians or parties; we are trying to support a process – an inclusive process," said Earnest.

The State Department also faced repeated questions about why it had not changed its position on aid to the Egyptian military in light of its attack on protestors.

"This is not about responding to one day," replied spokeswoman Jen Psaki. "Certainly there are have been some significant bumps in the road but our focus is getting back on a path to democracy."

She said there were also US national security and regional stability interests in continuing to provide aid to Egypt.

Nato also joined the public condemnation, a possible sign that western military leaders are losing patience with the generals in Cairo.

"I am deeply concerned by the situation in Egypt, and the continuing reports of bloodshed. I deplore the loss of life," said Nato secretary general Andres Fogh Rasmussen in a statement.

"Egypt is an important partner for Nato through the Mediterranean Dialogue. I call on all sides to exercise restraint and refrain from violence and to work to restore the political process."

Previously, Pentagon officials have been in regular contact with the counterparts in the Egyptian military, supplementing regular dialogue with the State Department and congressional leaders such as Republicans John McCain and Lindsey Graham.

But such contact has seemingly failed to persuade the interim government to tread lightly in dealing with pro-Morsi supports, raising the prospect that the US may revisit its strategy toward Egypt.

For the moment, the White House insists it is simply continuing to evaluate its support, while stepping up its threat to sever ties if the military does not change tack.

"They have made promises. When the interim government took control of the country, they promised this was only an interim step," Earnest said.

"That's a promise they made and that's a promise we're going to encourage them to keep."

On Wednesday, McCain publicly criticised the administration's handling of the crisis, tweeting: "As we predicted and feared, chaos in #Cairo. Sec Kerry praising the military takeover didn't help."