12:41: The press conference has now ended.
12:40: "We must prevent the Russian troops from moving deeper into Ukraine. We understand how difficult it is for Ukrainian troops, but we must win," says Mr Turchynov.
12:39: Mr Ban earlier told the Ukrainian president how encouraged he was to see the reintroduction of Russian as an official language, but Mr Turchynov jumped in to correct him to insist that Ukraine only has one language.
12:38: Ukrainian troops in Crimea are still under orders to protect their bases and ships, Mr Turchynov adds.
Olexiy Solohubenko, Ukraine anayst tweets: Visa explains blocking cards of "Russia", SMP Bank, Sobinbank and InvestCapitalBank to "align its activities in compliance with U.S. law"
12:35: "Ukraine will never accept the seizure of its territory. We are ready for talks with Russia in any kind of format, but their troops must leave Ukraine," says acting Ukrainian President Turchynov.
12:34: Moments before, Russian President Vladimir Putin ratified the treaty to absorb Crimea into Russia.
12:32: More from Mr Ban: I am not in a position to predict any possible reactions of possible course of action by Putin, but I have conveyed my serious concerns that this situation must be resolved through dialogue and in a peaceful manner. His (Mr Putin's) government and Ukraine should resolve this issue peacefully. It has become a global issue.
12:31: In another development, the US says it is preparing military exercises in Poland that could involve Poland, the Czech Republic, Hungary, Slovakia, Romania, Bulgaria and the Baltics, the US ambassador to Poland was quoted by Reuters as saying in an emailed press release. He said the exercises would take place at the Lask air base, central Poland.
12:30: The UN secretary-general says all sides should refrain from engaging in highly inflamed rhetoric, which he says is unhelpful and may lead to "an uncontrollable situation".