John Kerry, US secretary of state, will use visit to Ukrainian capital to consider options amid frustration in the west at failure to find diplomatic solution

The high-stakes poker game over east Ukraine continued on Monday, with US officials floating the idea of sending lethal military assistance to the government in Kiev, while Russia-backed separatists announced a plan to mobilise a 100,000-strong army.

The US secretary of state, John Kerry, is due in Kiev on Thursday, and while no major announcements are expected this week he will use the visit to take the temperature in the Ukrainian capital as the US administration weighs up the move.

Lt Col Vanessa Hillman, a Pentagon spokesperson, said the administration’s focus remains on pursuing a solution through diplomatic means, but added: “We are always evaluating other options that will help create space for a negotiated solution to the crisis.”

Hillman said the administration continues to assess how best to support Ukraine while declining to discuss what she called the details of internal policy discussions. A senior administration official told the Associated Press on Monday that President Barack Obama is reconsidering sending lethal assistance to Ukraine, but remains wary of the risks of provoking a proxy war between the US and Russia.

Despite an influx of heavy weaponry from Russia to the separatists, the Obama administration has balked at a proportionate increase to the Ukrainian government, for fear of provoking an even more aggressive Russian response. The $118m worth of aid the US military has so far committed to sending Kiev’s military and border guard includes night-vision goggles, personal body armour and field medicine kits. About a third of that total has arrived, Hillman said.

However, there is frustration in the west at the failure to find a diplomatic solution to the conflict. The latest round of peace talks in Minsk over the weekend failed to get off the ground, with the rebel leadership keen to renegotiate the terms of a ceasefire agreed in September.

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The agreement sought to freeze the conflict at the position of the front lines as they stood in September, but since the new year, a new rebel offensive has begun, with the separatist forces taking control of Donetsk airport and attacking the key railways junction of Debaltseve, which has come under relentless artillery fire in recent days.

The leader of the Donetsk separatists, Alexander Zakharchenko, said on Monday he planned a wave of mobilisation to create an army of 100,000 men, though it was unclear how such mobilisation could be enforced. Some worried it could provide cover for a further injection of Russian troops.

Officially, Moscow continues to deny it is a party to the conflict, and has repeatedly called for Kiev to sit down with the separatists and talk. On the ground, however, the continuing Russian support is clear, as the separatists appear to have an inexhaustible supply of Grad missiles and other weaponry. There have also been sightings, including one by the Guardian, of Smerch multiple missile launchers in recent days, a sophisticated weapons system with a range of up to 100km, which previously has not been used by the separatists, suggesting it has also been sent from Russia in recent weeks. Ukraine’s president has claimed there are 9000 Russian soldiers on his soil, while the Kremlin continues to claim any Russians in east Ukraine are volunteers.

A report released on Monday by eight former top US officials calls on the US to provide arms to Ukraine. “Russian success would fatally undermine Ukraine’s stability and embolden the Kremlin to further challenge the security order in Europe,” says the report, which recommends giving Ukraine $1bn of military assistance in 2015, and a further $1bn each year in 2016 and 2017.

The German chancellor, Angela Merkel, has taken a tougher line on Putin in recent months and has come out strongly in support of sanctions against Russia, but on Monday unequivocally ruled out military assistance.

“Germany will not support Ukraine with guns and weapons,” said Merkel, speaking in Budapest. “We are putting all our bets on sanctions and doing our best to find a diplomatic solution.”

There is an increasing feeling in Kiev that as long as Russia proves willing to funnel fighters and funds to the separatists fighters, a military solution to the problem is impossible. Instead, Kiev wants to focus on not giving up further ground. It appears the Kremlin may have its eye on several key points for the infrastructure of a future “breakaway state” in east Ukraine, such as the railway junction of Debaltseve and the port of Mariupol. Ukraine has stopped paying pensions and other payments in the region after losing full control, and the Kremlin has little desire to fund east Ukraine from its own coffers.

“Ukraine should probably not be given weaponry to take back Donbass territory as that could trigger the third world war,” said MP Victoria Voytsitska. “But Ukraine needs to be armed to protect the territory it has left.”

The official reaction in Russia to reports that the United States could supply arms to Ukraine was muted, with the foreign ministry not making any official statement. But various lawmakers condemned the news as a sign Washington was escalating the conflict in eastern Ukraine.

“If the US administration makes such a decision, it will lead to irrevocable consequences,” Viktor Zavarzin, a member of the Russian parliament’s defence committee, told state news agency RIA Novosti. “We’re going to hope that the United States will be rational when making a final decision on this.”

Another member of the defence committee, Frants Klintsevich, told journalists on Monday that Washington was trying to draw Russia directly into the conflict in Ukraine with its statement, adding: “We know very well that the United States has been delivering arms to Ukraine for a long time already.”

Russia has accused the US of orchestrating the Maidan coup and repeatedly accused the west of backing Ukraine’s forces in the east, with Putin talking about rebels fighting a “Nato legion” in the area and the foreign ministry repeatedly speaking about “foreign mercenaries” fighting alongside Ukrainian troops. There are a number of a foreigners fighting with Ukraine’s volunteer battalions, but there has been no evidence of any major influx of mercenaries. However, with the mood now changing in Washington, claims of US military assistance may now become a self-fulfilling prophecy.

US state department spokeswoman Jan Psaki said: “Our focus remains on a political and diplomatic solution. I don’t think anybody wants to get in a proxy war with Russia. And that is not the objective, our objective here is to change the behavior of Russia.”

Meanwhile, eight senior American defence experts released a report on Monday calling for the US to provide Ukraine with $1bn in military assistance as soon as possible and $1bn more in both the fiscal year of 2016 and 2017. The group also called for fellow Nato members to provide equipment and weapons to Ukraine, especially those who have have former Soviet equipment that would be compatible with Ukraine’s current arms stock.

The report was written by people including Michele Flournoy, the former under secretary of defence who took herself out of the running to replace outgoing defence secretary Chuck Hagel, and James Stavridis, who served as the top Nato military commander.

The group advised the US to help create a defense situation that, combined with existing economic sanctions, Russia would consider too costly to pursue. The group also said that it does not at this time support an on-the-ground US or Nato presence.