Image copyright EPA Image caption The US leads a coalition carrying out air strikes on so-called Islamic State in Syria and Iraq

Dozens of US State Department officials have signed an internal memo protesting against US policy in Syria and calling for targeted military strikes against President Bashar al-Assad's government.

They argue the current approach is working against the Syrian opposition and helping Mr Assad to stay in power.

It was signed by 51 mid-to-high level officials who've been involved in implementing the administration's Syria strategy.

It is not unusual for internal "dissent cables" to be filed through State Department channels.

However, it is extremely rare to have this number of diplomats voice opposition to a White House position.

A State Department spokesman acknowledged receipt of the memo but declined to comment on its contents.

But an official familiar with the letter told the BBC that it was sent "because the status quo is not sustainable".

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The document urges a credible threat of military action against the Assad government. Otherwise, it says, Damascus will feel no pressure to negotiate with the rebels and continue violating the ceasefire.

The large number of signatories is a real "indictment of the policy from the people who are trying to implement it," says Robert Ford, a former ambassador to Syria who resigned in 2014 out of frustration with the administration's hands-off approach to the conflict.

"What they're saying is we cannot achieve our objectives" for a sustainable ceasefire or a negotiated peace deal, he told the BBC.

"People are very frustrated with the barrel bombs in Aleppo, the targeting of hospitals, the flow of refugees."

The Obama administration is pushing for a political solution to the civil war but has mostly steered clear of military involvement, focusing instead on the fight against the Islamic State group in Syria.

The letter follows the apparent collapse of the joint US-Russian peace process.

There have been violations of the ceasefire by both the opposition and the government.

But President Assad has openly defied the truce, and his forces, backed militarily by Iran and Russia, seem intent on regaining strategic territory such as the crucial city of Aleppo.

Image copyright AFP Image caption Syrian cities such as Aleppo have been left in ruins by years of war

Moscow argues that it is supporting strikes against jihadist insurgents not covered by the ceasefire.

But Secretary of State John Kerry, who has pushed opposition groups to lay down their arms, is growing increasingly frustrated as the Syrian regime continues to change facts on the ground while he calls for diplomacy.

"The United States is not going to sit there and be used as an instrument that permits a so-called ceasefire to be in place while one principal party is trying to take advantage of it to the detriment of the entire process," he said recently.

Mr Kerry has pressed the Obama administration for tougher action against the Assad government in order to force it to the negotiating table, and signatories to the dissent letter believe he'll be sympathetic to their suggestions, according to sources involved in the process.

But that's unlikely to sway President Obama, who is wary of being drawn into another Mideast conflict after the messy results of US intervention in Iraq, Afghanistan and Libya. He's also reluctant to risk further tensions and possible military confrontation with Russia.

However, his possible successor, presumptive Democratic presidential candidate Hillary Clinton, has argued for a more assertive policy in Syria, including stronger support for non-Islamist rebels.

Perhaps the letter is aimed as much at her as it is at Mr Kerry.