Nato leaders have descended on the Welsh resort of Celtic Manor for a two-day summit, which formally starts with a meeting about Afghanistan but will be dominated by discussion on Ukraine and the threat of Islamic State (Isis) extremists in Iraq and Syria.

Ukraine's president, Petro Poroshenko, will brief leaders on Thursday on his agreement with the Russian president, Vladimir Putin, on the outlines of a peace agreement in Ukraine.

A British official said: "The meeting will provide leaders with the opportunity to hear president Poroshenko's assessment of the latest situation on the ground and his discussions with President Putin. It will also send a clear signal of their support for Ukraine's sovereignty and that the onus is on Russia to de-escalate the situation."

The Russian foreign minister, Sergei Lavrov, warned on Thursday that Ukraine's Nato ambitions were threatening to derail peace talks in eastern Ukraine. In televised remarks, Lavrov said statements by senior government officials in Kiev that they would be seeking to join Nato were "a blatant attempt to derail all the efforts" to seek a peaceful solution to the crisis.

The organisers had hoped that summit would help mark a new era in Afghanistan at the end of the alliance's combat mission, by welcoming Hamid Karzai's successor as the country's new president. But the result of April's election is still a matter of dispute between the rival candidates Abdullah Abdullah and Ashraf Ghani, amid growing insecurity in the country.

The failings of Nato's mission in Afghanistan were underlined by more violence on Thursday as Taliban insurgents detonated two truck bombs in the central town of Ghazni, killing 18 people.

The first item on the formal agenda at the summit is a heads of government meeting on Afghanistan, where leaders will discuss how Nato can support the country after the last troops leave at the end of this year.

Speaking before the meeting the Nato general secretary, Anders Fogh Rasmussen, said it was "vital to see a conclusion to the electoral process in Afghanistan".

He also tried to put a positive gloss on Nato's campaign. "We will prepare a new chapter in our relationship with Afghanistan, as our combat mission draws to a close," he said.

Rasmussen added: "We have done what we set out to do. We have denied safe haven to international terrorists. We have built up capable Afghan forces of 350,000 troops and police. So our nations are safer, and Afghanistan is stronger.

"We have planned a new mission to train, advise and assist the Afghan forces from next year and it will be launched once we have the legal arrangements in place.

Later in the summit Nato is expected to agree to bolster its eastern defences and buttress support for Ukraine. It is set to approve the setting up of a "spearhead" rapid reaction force, potentially including several thousand troops, that could be sent to a hotspot in as little as two days, officials say.

Eastern European members, including Poland, have appealed to Nato to permanently station thousands of troops on its territory to deter any possible Russian attack. But other members have spurned that idea, partly because of the expense and partly because they do not want to break a 1997 agreement with Russia under which Nato committed not to permanently station significant combat forces in the east.

Instead Nato leaders will agree to pre-position equipment and supplies, such as fuel and ammunition, in eastern European countries, with bases ready to receive the Nato rapid reaction force if needed.

Nato has said it has no plans to intervene militarily in Ukraine, which is not a member.

The crisis in Iraq and Syria is not on the formal agenda but is expected to dominate discussions at the sidelines, as Barack Obama and David Cameron attempt to build an international coalition for tackling Isis militants.

Writing in the Times on Thursday they said: "When the threats to our security increasingly emanate from outside Nato's borders, we must build more partnerships with others who share our values and want a tolerant and peaceful world. That includes supporting the partners who are taking the fight to Isil [Islamic State] on the ground, as we have done by stepping up support for Kurdish and Iraqi security forces."

They added: "We meet at a time when the world faces many dangerous and evolving challenges. To the east, Russia has ripped up the rulebook with its illegal, self-declared annexation of Crimea and its troops on Ukrainian soil threatening and undermining a sovereign nation state.

"To the south, there is an arc of instability that spreads from north Africa and the Sahel, to the Middle East."