Nicola Sturgeon has said she was "deeply frustrated" by a meeting with the prime minister to discuss Brexit.
The Scottish first minister joined her Welsh and Northern Irish counterparts in Downing Street for the talks with Theresa May.
But Ms Sturgeon said the discussions had left her no clearer about the UK government's thinking.
Downing Street has warned the leaders of the devolved administrations against "undermining" Brexit negotiations.
The prime minister's official spokesman said: "We have been very clear that we should be working together to secure the best possible deal for the whole country.
"We expect representatives of the devolved administrations to act in that way and to in no way undermine the UK's position."
Asked about calls for different parts of the UK to be able to opt in or out of the European single market, Downing Street said a united UK negotiating position was "vital to protect the UK's interest as a whole".
Ms Sturgeon and the Scottish government's Brexit minister, Michael Russell, spent about two hours inside Downing Street before emerging to answer questions from the media.
She told BBC Scotland: "We discussed the UK's negotiating position in general, but it is safe to say we got no more information or detail on that than we had before we went into the meeting, and I got the strong sense the UK government itself doesn't know what it is trying to achieve.
"That is why many parts of the meeting were deeply frustrating, because we felt as if we weren't getting any greater insight into the thinking of the UK government."
Scotland and Northern Ireland voted to remain in the EU in the referendum in June, while Wales, like England, voted to leave.
Ms Sturgeon and Welsh first minister Carwyn Jones have both called for votes on the Brexit strategy, saying Article 50 should not be triggered until there is an agreed approach.
The two first ministers have stressed their desire to see continued participation in the single market - a situation that could be at risk if the UK pursued a so-called "hard" approach to Brexit.
Ms Sturgeon has said she will bring forward specific proposals for a so-called flexible Brexit that would keep the nation in the single market even if the rest of the UK was not part of the trading agreement, in the next few weeks.
Downing Street is proposing that Brexit Secretary David Davis chair a new forum bringing together representatives from the devolved nations before the prime minister triggers Article 50 of the Lisbon Treaty, formally starting the two-year Brexit process, by the end of March next year.
Ms Sturgeon said discussions on Scotland being given a say in the Brexit negotiations were "slightly further forward".
She added: "We have still got a lot of work to do, and I think it is incumbent on the UK government to inform that work by being much more open about what they are trying to achieve.
"I am determined, for my part, to do everything I can to work within this process as far as I can to protect Scotland's interests.
"My frustration is that I am hearing warm words from the UK government, but not yet seeing those warm words backed up by substance or action."
Ms Sturgeon also said she was not prepared to sit back and "watch Scotland being driven off a hard Brexit cliff edge", because the implications for jobs, the economy and living standards were too serious.
She insisted she was determined to find a way of the EU referendum results in England, Wales, Northern Ireland and Scotland being respected.
And she said she was not "bluffing" about the possibility of a second independence referendum, adding: "If all we get from the UK government is the door getting closed in our face, then I am absolutely determined that Scotland shouldn't be taken off that hard Brexit cliff edge without at least having the opportunity to choose a better future."