* Taliban pledge more suicide attacks, assassinations
* ‘Operation Omari’ named after former Taliban leader
* US embassy earlier issued warning of possible attack
* At least five border police killed in insider attack 

The Taliban announced the start of their spring offensive on Tuesday, pledging to launch large-scale offensives against government strongholds backed by suicide and guerrilla attacks to drive Afghanistan's Western-backed government from power.

The announcement of the start of ‘Operation Omari’, named after the late Taliban founder Mullah Mohammad Omar, came just days after Secretary of State John Kerry visited Kabul and reaffirmed US support for a national unity government led by President Ashraf Ghani.

‘Jihad against the aggressive and usurping infidel army is a holy obligation upon our necks and our only recourse for re-establishing an Islamic system and regaining our independence,’ the Taliban said in a statement.

The insurgency has gained strength since the withdrawal of international troops from combat at the end of 2014 and the Taliban are stronger than at any point since they were driven from power by US-backed forces in 2001.

As well as suicide and tactical attacks, the offensive would include assassinations of ‘enemy’ commanders in urban centres, the Taliban said.

‘The present operation will also employ all means at our disposal to bog the enemy down in a war of attrition that lowers the morale of the foreign invaders and their internal armed militias,’ they said.

In line with recent statements, the militants also said they would establish good governance in areas they controlled and avoid civilian casualties and damage to infrastructure.

The seasons have long shaped violence in Afghanistan with fighting easing off in the winter, when mountain passes get snowed in, and picking up again in the spring and summer.

How far the announcement will lead to an immediate escalation in fighting, which caused 11,000 civilian casualties last year, remains unclear. However, NATO and Afghan officials have said they expect very tough combat in 2016.

US EMBASSY WARNING

While there was no sudden surge in violence, officials said at least five members of Afghanistan's border police had been killed overnight when one police officer turned his weapon on his colleagues.

The ‘insider attack’ happened at a frontier post in southern Kandahar province and the attacker fled into Pakistan. The Taliban claimed seven police had been killed and that the attacker had gone over to join the Islamist group.

Hours before the Taliban announced their offensive, the US embassy in Kabul issued an emergency warning to US citizens, saying it had received reports that insurgents were planning attacks on a major hotel in Kabul.

Heavy fighting has continued for months across Afghanistan, from Kunduz, the northern city that fell briefly to the insurgents last year, to Helmand province bordering Pakistan in the south.

In Helmand, where thousands of British and American troops were killed or wounded fighting the Taliban, government forces have pulled back from many areas and are struggling to hold centres close to the provincial capital, Lashkar Gah.

Understrength Afghan security forces, struggling with heavy casualties and high desertion rates and short of air power, transport and logistical support, have struggled in their first year fighting largely alone.

According to NATO commanders, the Taliban exert control over only six percent of Afghanistan but up to a third of the country is at risk from the insurgents and government forces control no more than 70 percent of territory.

US General John Nicholson, who took over as commander of international troops in Afghanistan last month, is conducting a strategic review, including plans to cut US troops in Afghanistan from 9,800 to 5,500 by the end of the year.

Unless the plan is changed, the reduction would mean the end of most of NATO's training and assistance operation, leaving the remaining US troops focusing on counter-terrorism operations against radical groups such as Islamic State.