Murder rates have increased sharply across the US in 2015, with at least 30 cities reporting a rise in violence.

Experts say factors may vary from city to city, but no-one really knows what has caused the trend, which was first identified this summer.

The latest figures obtained by the New York Times show Milwaukee has seen the biggest spike, with a 76% increase.

It is followed by St Louis, Missouri, which has seen a 60% increase, and Baltimore, Maryland, with a 56% rise.

Some senior police officials say that heightened scrutiny of use of force by officers may have caused them to withdraw from everyday policing and emboldened criminals.

But others say an increase in gun ownership, a spike in gang-related killings and more young people settling disputes with guns may be contributing factors.

Why has the murder rate spiked? By Ashley Gold, BBC NewsMichael Brown's killing in Ferguson last summer and unrest that followed has sparked debate over police and race relations in the US

While it is unclear what has led to the sharp increase, some say it can be attributed to "the Ferguson effect" - the theory that police officers have been withdrawing from everyday enforcement due to fears they could be charged.

It follows a nationwide debate over the police's use of force and race relations in the wake of the fatal shooting of black teenager Michael Brown by a white police officer in Ferguson, Missouri, last year.

But others argue that while this may be the case for some local populations in St Louis and Baltimore where racial tensions have been high, it does not explain the nationwide surge.

Violent crimes are mostly driven by social factors such as the size of the youth population, the amount of socioeconomic disadvantage and social disorganisation in a given city.

Read more: Why has the murder rate in some US cities suddenly spiked?

The latest figures show the number of murders in Milwaukee increased from 59 last year to 104 so far in 2015.

In St Louis, it rose from 85 last year to 136 so far this year, and in Baltimore shot up from 138 last year to 215 by the summer of 2015.

In Washington, there have been 105 murders - a 44% increase from last year.

The numbers have also jumped in other cities, including New Orleans, Chicago, Kansas City, Dallas, New York and Philadelphia, though to a lesser extent.