The top US diplomat in Kabul warned on Wednesday that Pakistan posed a bigger security challenge to America and the world than Afghanistan, as Islamabad grappled with the latest terrorist attack on its soil and the escalating Taliban insurgency on its north-western border. Christopher Dell, who currently runs the US embassy in Kabul, was speaking in the aftermath of the attack on the Sri Lankan cricket team in Lahore and the news that Pakistani Taliban groups had formed a common front to attack Nato troops in Afghanistan, in what is widely expected to be a bloody and possibly decisive summer this year. "From where I sit [Pakistan] sure looks like it's going to be a bigger problem," Dell said in an interview in the heavily fortified US embassy in Kabul. "It is certainly one of those nuclear armed countries the instability of which is a bigger problem for the globe. "Pakistan is a bigger place, has a larger population, its nuclear-armed. It has certainly made radical Islam a part of its political life, and it now seems to be a deeply ingrained element of its political culture. It makes things there very hard." Fears over Pakistan's ability to cope with the rise of violent religious extremism were intensified by claims yesterday that police in Lahore had abandoned the Sri Lankan cricketers whom they were supposed to be protecting when gunmen opened fire on Tuesday. Surveillance footage showed three of the attackers walking down the middle of a street, apparently under no pressure. But Pakistani officials pointed out that six police officers died in the attack. Senior officials in the UK Foreign Office and the Obama administration have privately expressed concern that Pakistan could prove to be more of a danger to global peace and security in the long run than Afghanistan, because of its nuclear weapons and its highly politicised and Islamicised secret service, the Inter-Services Intelligence agency (ISI). Barack Obama is particularly alarmed at the decline in Pakistan's stability, and appointed a special envoy, Richard Holbrooke, to Afghanistan and Pakistan to coordinate diplomatic efforts. In a reflection of rising anxiety in Washington, Dell expressed those concerns openly. Dell, who is serving as the US charge d'affaires in Kabul after a similarly outspoken stint as ambassador in Zimbabwe, said there were signs the rate of infiltration of insurgents across the frontier from Pakistan's semi-autonomous tribal areas had increased in recent days. He said it was possible the increase was a result of ceasefire deals agreed by militants and the Pakistani government. "Every time the Pakistanis have signed a peace deal, two things happen," Dell said. "There is an uptick in the fighting on this [the Afghan] side, and the peace deals have fallen apart quickly. We think we've already seen an increase of fighters crossing the border." The epicentre of the problem is Pakistan's Federally-Administered Tribal Areas (Fata) which have become a stronghold for an array of jihadist groups including al-Qaida and various splinters of the Taliban. The Guardian reported on Tuesday that three warlords in the Fata had settled their differences, formed a group calling itself Shura Ittihad-ul Mujahideen, or Council of United Holy Warriors, and had agreed to focus their efforts on launching attacks in Afghanistan. Major General John MacDonald, the new deputy commander of US forces in Afghanistan, told the Guardian the insurgents were "most dangerous when they begin to collaborate with one another". He predicted that the coming surge in the number of coalition troops in Afghanistan would lead to an increase in fighting. "So yes this summer you will see more violence," he said. "We're just about to kick a beehive."