KYIV, Jan 19 (Reuters) - U.S. Secretary of State Antony Blinken said on Wednesday that Russia could launch a new attack on Ukraine at "very short notice" but Washington would pursue diplomacy as long as it could, even though it was unsure what Moscow really wanted. On a visit to Kyiv to show support for Ukraine, the top U.S. diplomat said Ukrainians should prepare for difficult days. He said Washington would keep providing defence assistance to Ukraine and renewed a promise of severe sanctions against Russia in the event of a new invasion. The Kremlin said tension around Ukraine was increasing and it was still waiting for a written U.S. response to its sweeping demands for security guarantees from the West. The pessimistic statements highlighted the gulf between Washington and Moscow before talks between Blinken and Russian Foreign Minister Sergei Lavrov on Friday that a Russian foreign policy analyst called "probably the last stop before the train wreck". Blinken said he would not present a written response on Russia"s proposals to Lavrov in Geneva but promised "relentless diplomatic efforts to prevent renewed aggression and to promote dialogue and peace". He said a Russian build-up of tens of thousands of troops near Ukraine"s borders was taking place with "no provocation, no reason". "We know that there are plans in place to increase that force even more on very short notice, and that gives President (Vladimir) Putin the capacity, also on very short notice, to take further aggressive action against Ukraine," he said. He did not spell out how quickly Russia might move. Independent security analysts say they do not believe Moscow has so far assembled the logistics and medical units it would need to launch an immediate attack. Russia has also moved troops to Belarus for what it calls joint military exercises, giving it the option of attacking neighbouring Ukraine from the north, east and south. Eight years ago it seized Crimea and backed separatist forces who took control of large parts of eastern Ukraine, but it has consistently denied any intention of invading now. Kremlin spokesman Dmitry Peskov said Western weapons deliveries to Ukraine, military manoeuvres and NATO aircraft flights were to blame for rising tensions around Ukraine. Russian Deputy Foreign Minister Sergei Ryabkov said he did not believe there was a risk of a large-scale war and reiterated that Moscow had no plans to attack, strike or invade Ukraine. His comments prompted a rally in Russian and Ukrainian government bonds, hammered in recent weeks by the escalating tensions. Ukrainian Foreign Minister Dmytro Kuleba accused Russia of trying to sow panic in Ukraine; President Volodymyr Zelenskiy said the government was doing everything to resolve the situation through diplomacy and citizens should remain calm. "Keep a cool head, maintain confidence in your strength, in your army and in our Ukraine," he said. Russia says it feels menaced by Kyiv"s growing ties with the West. It wants to prevent Ukraine ever joining NATO and for the alliance to pull back troops and weapons from eastern Europe. "It"s not clear what Russia"s central demand is or is not. They"ve put a number of things on the table," Blinken said. "Some of them are clearly absolute non-starters like closing NATO"s door to new members," he said. Washington remained ready to discuss anything that enhanced "everyone"s security on a reciprocal basis", he said, including arms control, risk reduction, and the scale and scope of military exercises. After Friday, he added, the diplomatic possibilities might become clearer. But Vladimir Frolov, a former Russian diplomat who is now a foreign policy analyst, said Moscow would not be appeased by the U.S. and NATO offer of arms control talks and was seeking a much more sweeping rearrangement of Europe"s security order. "The Lavrov-Blinken meet is probably the last stop before the train wreck. But hopes are dim, the positions are incompatible," said Frolov. Describing Russia"s military deployment in Belarus as a "huge escalation", Frolov gave a dire assessment of the crisis. "I think barring a U.S. surrender and their delivering Ukraine to Russia, some kind of a military option is all but inevitable now." SANCTIONS THREAT In the United States, Republican senators said they were working with Democrats to come up with a joint proposal for possible sanctions on Russia to prevent another attack on Ukraine. Separately, the White House told the U.S. chip industry to be prepared for new restrictions on exports to Russia if Moscow attacks Ukraine, sources said, including potentially blocking the country"s access to global electronic supplies. Zelenskiy thanked Blinken in Kyiv for stepping up military assistance, which has included Javelin anti-tank missiles, after President Joe Biden"s administration last month approved an additional $200 million. Britain says it has begun supplying Ukraine with anti-tank weapons. Blinken said more assistance was scheduled in coming weeks, and it would be further increased if Russia invaded. Ryabkov called on the West to stop supplying Ukraine with weapons, the Interfax news agency reported. Blinken will hold talks in Berlin on Thursday with Germany"s foreign minister and the Quad grouping that also includes Britain and France. French President Emmanuel Macron said the European Union - which has not been involved until now in negotiations with Russia - must draw up proposals in the coming weeks for a new security deal that it would then thrash out with Moscow.