The US' almost-1.5 year-long campaign against ISIS is depleting the country's Air Force's bomb reserves.

The country is said to have fired 20,000 missiles and bombs in Syria and Iraq.

Even late on Sunday, the US-led coalition bombed Syria, killing members of the Syrian army.

"B-1s have dropped bombs in record numbers. F-15Es are in the fight because they are able to employ a wide range of weapons and do so with great flexibility. We need the funding in place to ensure we're prepared for the long fight," Air Force chief of staff General Mark Welsh said in the statement. 

Claiming that the need is "critical", Welsh said that the stock of ammo is "below our desired objective."

The US Air Force has put in a request for restocking the reserves of munitions. However, an official said to CNN that the restocking process would take "up to four years from time of expenditure to asset resupply".

"The precision today's wars requires demands the right equipment and capability to achieve desired effects. We need to ensure the necessary funding is in place to not only execute today's wars, but also tomorrow's challenges," the official said. 

The US has raised the number of strikes to half as compared to one-third in 2014, the US Air Force's publication states. 

US President Barack Obama is battling criticism at home as Republicans ask for more aggressive strategy to battle Daesh after the mid-November Paris attacks and the California mass shooting, which is being considered as a "terror attack" as the couple are being assumed to be self-radicalised. 

The death toll in Syria currently stands at around 250,241–340,124, according to the Syrian Observatory for Human Rights, while the United Nations estimates it to be around 220,000.

Britain has extended its air strikes to Syria from Iraq where as France has sent its nuclear-powered ship, Charles de Gaulle, which is an aircraft carrier, towards eastern Mediterranean.